0. Basic Concepts of Photometric Quantities

In everyday language, we often say things such as “this lamp is very bright,” “the desk is very bright,” or “the screen is very bright.”

However, in physics and lighting engineering, the word “bright” cannot be represented by just one quantity, because it may refer to different things:

• how strongly a light source emits in a particular direction;
• how much light a source emits in total;
• how much light falls on a particular surface;
• how much light a surface itself emits outward;
• how bright a surface appears to the human eye from a particular direction.

Here, the subscript $V$ denotes visual, meaning that the quantity is weighted according to the visual sensitivity of the human eye.

0. The First Problem Photometry Solves: Not All Light Appears Equally Bright

Photometry studies not simply energy, but light as perceived by the human eye.

For example, for the same radiant power of $1\ \mathrm{W}$, green light generally appears brighter to the human eye than red or violet light.

This is because the eye has different sensitivities to different wavelengths.

Under photopic vision conditions, the eye is most sensitive to yellow-green light near approximately $555\ \mathrm{nm}$.

Thus photometric quantities such as luminous flux, luminous intensity, illuminance, and luminance are not simply energy quantities. They are weighted by the human visual sensitivity function.

Roughly speaking:

Radiometry asks: how much energy does the light physically carry? Photometry asks: how strong does that light appear to the human eye?

1. Luminous Intensity: How Strongly a Light Source Emits in a Particular Direction

1.1 Why Begin with “Direction”?

A lamp does not necessarily emit uniformly in all directions.

A flashlight concentrates most of its light forward; a candle emits approximately in all directions; a desk lamp directs more light toward the desktop.

So let us first consider one direction:

how strongly does the source emit in that particular direction?

This quantity is called luminous intensity.

1.2 Solid Angle: the Three-Dimensional Analogue of an Angle

In plane geometry, we use an angle to describe a range of directions.

In three-dimensional space, we use a solid angle to describe a spatial range.

The unit of solid angle is the steradian, denoted by $\mathrm{sr}$.

You can imagine it as follows:

Place a light source at the center of a sphere of radius $r$.

Suppose a beam of light reaches a small surface area $A$ on the sphere.

The spatial range subtended by this area at the center is the solid angle:

If the radius is $1\ \mathrm{m}$, then an area of $1\ \mathrm{m^2}$ on the spherical surface subtends a solid angle of $1\ \mathrm{sr}$ at the center.

The total surface area of a sphere is $4\pi r^2$, so the solid angle of the whole space is:

1.3 Definition of Luminous Intensity

Luminous intensity describes:

the luminous flux emitted by a light source per unit solid angle in a given direction.

The formula is:

where:

• $I_V$: luminous intensity;
• $\Phi_V$: luminous flux;
• $\Omega$: solid angle.

The unit is:

that is,

Intuitively:

Luminous intensity tells us how concentrated and how strong the emitted light is in a particular direction.

Therefore, a flashlight may not have an especially large total luminous flux, but if it concentrates the light into a small angular range, its luminous intensity in the forward direction can be very large.

2. Luminous Flux: How Much “Visible Light” a Source Emits in Total

2.1 From Luminous Intensity to Luminous Flux

If luminous intensity tells us how strong the light is in a particular direction, then summing the light over all directions gives the total amount of light emitted by the source.

This total quantity is called luminous flux.

The symbol is:

and the unit is the lumen:

It represents:

the total amount of light emitted per unit time after weighting by the visual sensitivity of the human eye.

More intuitively:

Luminous flux tells us how much visually useful light a lamp emits in total.

2.2 Relationship Between Luminous Flux and Luminous Intensity

If the luminous intensity of a source varies with direction as $I_V$, then the total luminous flux is the integral over all directions:

If the source emits uniformly in every direction, so that the luminous intensity is the same in all directions, then:

This is because the total solid angle of space is $4\pi\ \mathrm{sr}$.

For example, if an isotropic source has a luminous intensity of $1\ \mathrm{cd}$, its total luminous flux is:

2.3 Luminous Flux Is Different from Power

Lamp packaging often lists values such as “$800\mathrm{lm}$” or “$1000\mathrm{lm}$.”

Here, $\mathrm{lm}$ means lumens.

It is neither electrical power nor ordinary power measured in $\mathrm{W}$.

For example:

• a $10\ \mathrm{W}$ LED lamp may be very bright;
• a $10\ \mathrm{W}$ incandescent lamp may be much dimmer.

This is because $\mathrm{W}$ describes electrical power consumption, while $\mathrm{lm}$ describes how much visible light is emitted.

Therefore, when comparing the lighting capability of lamps, $\mathrm{lm}$ is more direct than $\mathrm{W}$.

3. Illuminance: How Much Light Falls on a Surface

3.1 Why Is Luminous Flux Alone Not Enough?

Suppose a lamp has a fixed total luminous flux.

If the light is concentrated onto a book, the book will appear bright.

If the same amount of light is spread across an entire room, each square meter receives less light, and the surfaces appear less brightly illuminated.

Therefore, knowing only the total amount of light is not enough.

We also need to know over how large an area that light is distributed.

This leads to illuminance.

3.2 Definition of Illuminance

Illuminance describes:

the luminous flux incident on a surface divided by the illuminated area.

The formula is:

where:

• $E_V$: illuminance;
• $\Phi_V$: luminous flux incident on the surface;
• $A$: illuminated area.

The unit is the lux, denoted by $\mathrm{lx}$:

That is:

If a $1\ \mathrm{m^2}$ desktop uniformly receives $1\ \mathrm{lm}$ of luminous flux, its illuminance is $1\ \mathrm{lx}$.

3.3 Illuminance Describes the Illuminated Surface

Illuminance concerns the illuminated surface rather than the light source itself.

For example:

• illuminance on a desktop;
• illuminance on a blackboard;
• illuminance on the floor;
• illuminance on a wall.

Thus it is reasonable to say:

“the desktop has an illuminance of $500\ \mathrm{lx}$.”

But saying:

“this lamp has $500\ \mathrm{lx}$”

is usually imprecise.

A lamp emits luminous flux or has luminous intensity; a desktop receives illuminance.

3.4 Illuminance Formula for a Point Source

If a point source has luminous intensity $I_V$ in a particular direction, the illuminated surface is at a distance $r$, and the angle between the light ray and the surface normal is $\theta$, then the illuminance is approximately:

This formula contains two important factors.

First, illuminance decreases with distance:

This is because light spreads through space as it propagates.

Second, illuminance decreases when light strikes the surface obliquely:

This is because the same beam of light covers a larger surface area when it strikes obliquely.

Intuitively:

The farther the light travels, the more it spreads out. The more obliquely it strikes, the more it spreads out. The more spread out it becomes, the less light each unit area receives, and the lower the illuminance.

4. Luminous Exitance: How Much Light Is Emitted from a Surface

4.1 Illuminance and Luminous Exitance Are Opposite Perspectives

Illuminance considers:

light falling onto a surface.

Luminous exitance considers:

light leaving a surface.

Both involve luminous flux divided by area, but the direction is different.

4.2 Definition of Luminous Exitance

Luminous exitance is defined as:

the luminous flux emitted outward per unit surface area.

The formula is:

where:

• $M_V$: luminous exitance;
• $\Phi_V$: luminous flux emitted from the surface;
• $A$: area of the luminous surface.

The unit is:

Note that although luminous exitance and illuminance have the same dimensional unit, $\mathrm{lm/m^2}$, the unit $\mathrm{lx}$ is generally reserved for illuminance, while luminous exitance is usually written as $\mathrm{lm/m^2}$.

4.3 Example

Suppose a luminous panel has an area of $2\ \mathrm{m^2}$ and emits a total luminous flux of $1000\ \mathrm{lm}$.

If the emission is uniform, its luminous exitance is:

This means:

each square meter of the emitting surface sends out $500\ \mathrm{lm}$ of light.

5. Luminance: How Bright a Surface Appears from a Particular Direction

5.1 Why Do We Still Need Luminance?

Illuminance tells us how much light reaches a desktop, but it does not completely tell us how bright the desktop appears to the eye.

For example:

Under the same illuminance, white paper appears bright while black fabric appears dark.

This is because white paper reflects more light toward the eye, while black fabric reflects less.

Another example:

For the same total luminous flux, a lamp with a small emitting area may appear glaringly bright, while a lamp with a large emitting area appears much softer.

Therefore, we need a quantity that describes:

how much luminous intensity a surface emits toward the eye per unit apparent area in a particular direction.

This quantity is called luminance.

5.2 Definition of Luminance

Luminance describes:

the luminous intensity of a surface in a given direction divided by its projected area as seen from that direction.

The formula can be written as:

where:

• $L_V$: luminance;
• $I_V$: luminous intensity in the given direction;
• $A_\perp$: projected area seen from that direction.

If the actual surface area is $\mathrm{d}A$, and the angle between the surface normal and the viewing direction is $\theta$, then the projected area is:

Therefore, luminance may also be written as:

The unit is:

It is also commonly called a $\mathrm{nit}$:

5.3 Luminance Is the Quantity Closest to “How Bright It Looks”

Luminance is commonly used to describe:

• screen brightness;
• luminance of luminaire surfaces;
• luminance of illuminated advertisements;
• road-surface luminance;
• sky luminance.

For example, when a smartphone display is advertised as having a “maximum brightness of 1000 nit,” this means:

The higher the luminance, the more light enters the eye per unit projected area and per unit solid angle, and the brighter the surface usually appears.

5.4 Difference Between Illuminance and Luminance

Illuminance and luminance are easily confused.

A useful distinction is:

Illuminance: how much light falls on an object. Luminance: how much light comes from the object toward the eye.

For example:

A desk is illuminated by a desk lamp.

The desk lamp provides illuminance on the desktop:

The desk then reflects light into the eye, giving the desk a certain luminance:

If the desktop is white, it reflects more light and has higher luminance.

If the desktop is black, it reflects less light and has lower luminance.

Even under the same illuminance, surfaces made of different materials may have different luminances.

6. Logical Relationship Between the Five Quantities

These quantities can be connected by one logical sequence.

Step 1: Look at One Direction

How strongly does the source emit in a particular direction?

This is luminous intensity, measured in $\mathrm{cd}$.

Step 2: Sum Over All Directions

How much light does the source emit in total?

This is luminous flux, measured in $\mathrm{lm}$.

Step 3: Look at Light Falling on an Area

How much light does a surface receive per unit area?

This is illuminance, measured in $\mathrm{lx}$.

Step 4: Look at Light Emitted Outward from a Surface

How much light does a surface emit outward per unit area?

This is luminous exitance, measured in $\mathrm{lm/m^{2}}$.

Step 5: Look at How Bright the Surface Appears from a Particular Direction

What is the luminous intensity per unit projected area of a surface in a given direction?

or:

This is luminance, measured in $\mathrm{cd/m^{2}}$.

7. An Intuitive Summary

You can imagine light as water.

Luminous Flux Is Like the “Total Amount of Water”

How much light a lamp emits in total is analogous to how much water flows out of a faucet in total.

The unit is $\mathrm{lm}$.

Luminous Intensity Is Like “How Concentrated the Water Flow Is in a Particular Direction”

If water sprays in every direction, no single direction is especially strong.

If the water is concentrated through a nozzle in one direction, the flow in that direction becomes very strong.

The unit is $\mathrm{cd}$.

Illuminance Is Like “How Much Water Reaches Each Square Meter of Ground”

With the same amount of water, if it is poured onto a small area, the ground becomes very wet.

If it is spread over a large area, each square meter receives less water.

The unit is $\mathrm{lx}$.

Luminous Exitance Is Like “How Much Water Emerges from Each Square Meter of Ground”

Instead of water falling onto the ground, imagine that the ground itself sprays water outward.

The amount of water emitted per square meter is analogous to luminous exitance.

The unit is $\mathrm{lm/m^{2}}$.

Luminance Is Like “How Strong the Water Jet Appears from a Particular Direction”

For the same total amount of water, a small outlet appears very forceful, while a large outlet appears much gentler.

Similarly, for the same luminous flux, a small high-luminance source is more likely to cause glare, while a large low-luminance source appears softer.

The unit is $\mathrm{cd/m^{2}}$.

8. Distinguishing the Five Quantities in One Sentence Each

Luminous intensity:

How strongly a source emits in a particular direction.

Luminous flux:

How much visible light the source emits in total.

Illuminance:

How much light reaches a particular surface.

Luminous exitance:

How much light each square meter of a luminous surface emits outward.

Luminance:

How bright a surface appears when viewed from a particular direction.

The most important distinction is:

Luminous flux concerns the “total amount”; luminous intensity concerns “direction”; illuminance concerns the “incident area”; luminous exitance concerns the “emitting area”; luminance concerns “direction + projected area” and is the quantity most closely related to perceived brightness.

I. Choosing an Illuminance Meter

Using smartphone apps or engineering modes such as *#0*# for illuminance measurement is not recommended.

Their internal structures are different and they lack a cosine corrector.

A basic comparison is shown below:

A comparison made by someone else between a lux-meter app and a dedicated measuring instrument:

Luxmeter App vs. Measuring Device

II. Illuminance Measurement Procedure

1. Set the measurement range.
2. Arrange measurement points.
3. Record illuminance at each point.
4. Calculate evaluation parameters.

III. Two Questions Before Measurement

1. What Is the Purpose of the Measurement?

Reason:

The purpose can be divided into two categories:

evaluating a luminaire, or evaluating the lighting of a room.

Summary of Residential Study-Room Lighting Measurement Plans

Measurement Purpose	Evaluation Standard	Measurement Standard	Measurement Area	Measurement-Point Arrangement	Evaluation Parameters
Evaluate a desk lamp	GB/T9473-2022 Performance Requirements for Table Lamps for Paper Tasks	GB/T 5700-2023 Methods of Measurement for Lighting	Central area and total area of the desktop 【Approximate compliance evaluation of the desk lamp】	National-standard rectangular-area four-corner point method: divide the area into 100 mm square grids National-standard sector-area four-corner point method: see Appendix D of the desk-lamp standard, applicable to circular-standard lamps Four-corner point method: use an existing grid board, such as a tooling board, perforated board, or calligraphy felt, as a reference surface, divided into 100 mm square grids Main reading area of the desk 【easy / approximate evaluation of the desk lamp】 Four-corner point method: use 3K4 paper as a point-layout reference surface, divided into 99 mm × 105 mm grids	Within each measurement area: minimum horizontal illuminance illuminance uniformity (maximum : minimum)
Evaluate a ceiling light	GB/T50034-2024 Standard for Lighting Design of Buildings Recommended values: see Table 5	GB/T 5700-2023 Methods of Measurement for Lighting	Entire study room horizontal plane at H=0.75 m	Center-point method: if the room side length is less than 2.5 m, center-to-center point spacing should not exceed 0.5 m × 0.5 m; if the side length is greater than 2.5 m and less than 6 m, it should not exceed 1 m × 1 m.	Average illuminance illuminance uniformity (minimum : average)
Evaluate a “large floor lamp”	Multiple standards should be considered together Recommended values: see Table 6	GB/T 5700-2023 Methods of Measurement for Lighting	Entire desktop	Four-corner point method: use at least 6 sheets of A4 paper to create a point-layout reference surface, divided into 200 mm × 200 mm grids	Within each measurement area: average illuminance illuminance uniformity (minimum : average)
			Entire study room horizontal plane at H=0.75 m ceiling at the same elevation as the ceiling light	Center-point method: if the room side length is less than 2.5 m, point spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
			Ceiling at the same elevation as the ceiling light	Center-point method: if the room side length is less than 2.5 m, spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
			Walls, including large wardrobes and curtains	Center-point method: if the room side length is less than 2.5 m, spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
Evaluate study-room lighting	Multiple standards should be considered together Recommended values: see Tables 7 and 8	GB/T 5700-2023 Methods of Measurement for Lighting	Entire desktop	Four-corner point method: use at least 6 sheets of A4 paper as the reference surface, divided into 200 mm × 200 mm grids	Within each measurement area: average illuminance illuminance uniformity (minimum : average)
			Surrounding area, horizontal plane at H=0.75 m	Center-point method: use grids of approximately 250 mm
			Background area, plane at H=0.75 m ceiling at the same elevation as the ceiling light	Center-point method: if the room side length is less than 2.5 m, spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
			Ceiling at the same elevation as the ceiling light	Center-point method: if the room side length is less than 2.5 m, spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
			Walls, including large wardrobes and curtains	Center-point method: if the room side length is less than 2.5 m, spacing should not exceed 0.5 m × 0.5 m; if greater than 2.5 m and less than 6 m, no more than 1 m × 1 m.
			H=1.2 m or at the seated eye height of the user	—	Vertical point illuminance

Specific national standards:

GB/T9473-2022 — Performance Requirements for Table Lamps for Paper Tasks

GB/T50034-2024 — Standard for Lighting Design of Buildings

GB/T5700-2023 — Methods of Measurement for Lighting

2. Details of Evaluation Parameters

Illuminance performance evaluation parameters mainly include:

(1) calculating the average illuminance over the measured surface.

(2) Illuminance uniformity

Exception:

the national standard for desk-lamp performance uses minimum horizontal point illuminance.

IV. Preparations Before Measurement

1. When Using the Illuminance Meter for the First Time or After a Long Period of Nonuse, Confirm That It Is Working Properly

(1) Under completely artificial lighting conditions, choose any point and take a measurement.

(2) After the luminaire has been operating for 15 minutes, switch on the illuminance meter and remove the cap from the photometric head.

(3) The surface of the photometric head must be free of dust and stains.

Keep both the photometric head and illuminance meter stable.

Do not allow your body or hand to block the photometric head.

(4) Determine whether the meter is operating normally.

If the reading does not change within 1 minute, the meter is operating normally.

If the reading continuously fluctuates within 1 minute, press the “HOLD” button 6 times during that minute and record the six values.

Take the maximum and minimum values.

If

the illuminance meter is operating normally.

(5) Professional measuring instruments should be calibrated at least once per year.

In household use, the illuminance meter can instead be checked annually for drift.

Use the same desk lamp in the same position, ensuring identical distance and height.

After the lamp has been on for 15 minutes, select three fixed points at the center of the desk.

After the illuminance meter has been on for 5 seconds, measure each point three times.

Calculate the average and record it, then compare it with the previous year's data.

For a Class 1 illuminance meter, if each measured value differs from the initial value by no more than 1%, factory recalibration is unnecessary.

2. Precautions During Measurement

(1) During measurement, pay attention to the operating time of the illuminance meter and the exposure time of the photometric head.

(2) After completing one group of measurements and while preparing the next group, it is preferable to switch off the illuminance meter and replace the cap on the photometric head.

(3) After all measurements are complete, it is preferable to remove the batteries and periodically recharge them if applicable.

V. Measuring Desk-Lamp Illuminance

1. Purpose of Illuminance Measurement

To understand the illuminance conditions in the main reading area and evaluate the core optical performance of the luminaire.

2. Measurement Area

A

area on the desktop.

3. Measure Illuminance at Each Numbered Point

Divide the specified area into square grids with side length

and mark measurement-point numbers at the four corners of each grid.

Positioning the measurement points:

(1) If the desk-lamp brand provides a measurement grid sheet, use it.

(2) If no measurement grid is provided, position the lamp according to GB/T9473-2022:

a) Height of the light-emitting surface:

1. If the manual specifies the normal operating height of the emitting surface, follow the manual.

2. If the manual does not specify an operating height and the height is not adjustable, test at that fixed height.

3. If the manual does not specify an operating height, the height is adjustable, and the maximum height is less than 400 mm, test at the maximum height.

4. If the manual does not specify an operating height, the height is adjustable, and the maximum height is greater than or equal to 400 mm, test at a height of 400 mm.

b) Relative position between the desk-lamp emitting surface and the effective reading/writing working area:

1. If the manual specifies the normal operating position:

• If the vertical projection of the geometric center of the emitting surface lies outside the central rectangular area, test according to the normal operating position specified by the manual.

• If the vertical projection lies inside the central rectangular area, position the projection point at the midpoint of the upper edge of the central rectangle during testing.

2. If the manual does not specify the normal operating position:

If the relationship between the emitting surface and the effective working area is unspecified, position the vertical projection of the geometric center of the emitting surface at the midpoint of the upper edge of the central rectangle.

Perform measurements after the light source has reached stable operation.

4. Determination

(1) If the illuminance-meter reading at a numbered point is stable, record that reading.

(2) If the reading fluctuates, measure the point $2\sim3$ times by pressing the “HOLD” button $2\sim3$ times.

Use the average as the illuminance value of that point.

5. Calculate Illuminance Uniformity

After measuring all numbered points, determine the minimum horizontal illuminance in the central area and total area, then determine the corresponding maximum illuminance and calculate:

6. Compare with the National Standard

The maximum horizontal illuminance should not exceed

and the lamp should satisfy the recommended values in GB/T9473-2022:

Here,

5.3.1 Shielding and glare control:

the desk lamp should provide shielding and should not cause excessive glare.

For a desk lamp whose emitting surface is less than

above the desktop in its normal operating position, the surface luminance of every component visible to a seated observer should not exceed

Exception: If the seller provides illuminance data based on the sector-shaped area in the older 2017 version of the standard, which is common for circular desk lamps, then the sector-shaped measurement reference surface should be drawn according to Appendix A of GB/T9473-2022.

VI. Measuring Illuminance from a Ceiling Light

1. Measurement Purpose

A ceiling light provides the basic lighting for the entire room, i.e. general lighting.

2. Measurement Area

The entire room.

3. Measure Illuminance at Numbered Points

After all fixed and movable furnishings are in place, furniture is arranged, and curtains are closed, subtract the width occupied by wardrobes and divide the remaining study-room area into a rectangular grid, preferably square.

Use the center of each grid cell as a measurement point.

The spacing between measurement points should satisfy:

Maximum Measurement-Point Spacing for Different Measurement Areas

Note:

$m$ is the number of measurement points along the length direction, and $n$ is the number along the width direction.

(1) Use two measuring tapes and masking tape with relatively weak adhesion to mark the points.

(2) The measurement-point height is

so prepare a stand and mount the photometric head and illuminance meter on it with the photometric head facing horizontally upward.

At present, there is no national standard specifically for residential study-room lighting.

It is recommended to refer to GB/T50034-2024 — Standard for Lighting Design of Buildings.

4. Determination (Same as for the Desk Lamp)

(1) If the reading at a numbered point is stable, record the reading.

(2) If the reading fluctuates, measure the point $2\sim3$ times using the “HOLD” button and take the average.

5. Calculate Illuminance Uniformity

After measuring all numbered points, determine the minimum horizontal illuminance in the central area and total area, then determine the corresponding maximum illuminance and calculate:

6. Compare with Recommended Values

Note:

(1) A residence or study room is a familiar environment for the user rather than a public space with large numbers of people, such as a classroom.

Therefore, the requirement for illuminance uniformity does not need to be as strict.

(2) It is relatively difficult for a single ordinary diffuse ceiling light to achieve high illuminance uniformity over a typical room area.

Therefore, for general lighting in a study room provided by one ceiling light, especially after furniture has been placed, an illuminance uniformity of

is already very good.

If the uniformity reaches classroom level,

that is of course even better, provided that the result is not simply produced by very high luminous flux from excessive power.

Some brands provide illuminance-uniformity data for the main reading area of the desk.

This value is not meaningful here because the illuminance-uniformity definition used in this case is

Since there is no dedicated national standard for ceiling lights in this context, only recommended measured illuminance values are given below:

Recommended “Illuminance” Evaluation Parameters and Criteria for Ceiling Lights

Note:

Based on the user's required average reading/writing illuminance on the desktop and the price, choose a ceiling light providing one of the above illuminance levels.

If price is not a concern, the highest level may be selected directly.

VII. Illuminance Measurement Plan for a “Large Floor Lamp”

A “large floor lamp,” also commonly called a standing eye-care lamp or floor-standing eye-care lamp, is a relatively high-power floor luminaire with a large emitting area and an appearance somewhat similar to a streetlight.

1. Measurement Purpose

Measure the illuminance of general lighting throughout the room and on the entire desktop, as well as the average illuminance of the ceiling and walls and their illuminance uniformity.

(1) It can provide both general lighting and local lighting, so illuminance data should be measured for both the entire room and the desktop.

(2) Its main advantage is that it illuminates the ceiling to produce comfortable indirect lighting.

Therefore, the average illuminance of the ceiling and walls and their illuminance uniformity should also be measured.

2. Measurement Area

The entire room, the entire desktop, the ceiling, and the walls.

If large wardrobes or curtains cover a significant portion of a wall, treat them as equivalent to the wall surface.

3. Measure Illuminance at Numbered Points

(1) Measurement of general lighting illuminance in the study room: use the same method as for the ceiling light.

Notation:

$a$ — site length.

$b$ — site width.

$m$ — number of grid cells along the long-axis direction.

$n$ — number of grid cells along the short-axis direction.

(2) Wall and ceiling measurements: use the same general principle as for general-lighting measurements.

Where possible, arrange measurement points using square grids.

If the wall is relatively unobstructed, point placement and measurement are straightforward.

If furniture or other objects block the wall, divide the wall into several regions and place measurement points in each region using the spacing recommended by the national lighting-measurement standard.

(3) Ceiling measurement points: use the same point arrangement as for general lighting.

4. Determination (Same as for the Desk Lamp)

(1) If the reading is stable, record it.

(2) If the reading fluctuates, take $2\sim3$ measurements at each point and use the average.

5. Calculate Illuminance Uniformity

After measuring all numbered points, determine the minimum and maximum illuminance in each relevant area and calculate:

6. Compare with Recommended Values

There is no dedicated national standard for this type of “large floor lamp,” so the following values are recommendations based on measured illuminance after the room has been fully furnished and curtains are closed.

Recommended “Illuminance” Evaluation Parameters and Criteria for Large Floor Lamps

VIII. Reading and Writing Lighting in a Study Room

This is similar to the “large floor lamp” approach.

The difference is that the measurement area for general lighting in the study room should be divided into the background area, surrounding area, and desktop.

(1) The grid division and measurement-point arrangement for the background area are the same as for the general-lighting measurements used for ceiling lights and large floor lamps.

(2) The surrounding area should be divided, as far as possible, into

grids.

(3) The measurement-point arrangement for the desktop is the same as for the large floor lamp.

The average illuminance and uniformity for all these measurement areas are calculated in the same way as for the large floor lamp.

Refer to the following tables for evaluation criteria:

Revision: Adjusted According to GB50034-2024

Revision: Adjusted According to GB50034-2024

Note:

The recommended values in these two tables are intended for school-age minors with normal physiological conditions.

Adults under the age of 55 may use them as a reference.

If the goal is to evaluate whether daytime and nighttime illuminance under different weather conditions is adequate from the perspective of non-visual physiological effects of light, then vertical illuminance at the eyes in a seated position should also be measured.

The national standard recommends a height of

but the measurement may instead be performed at the actual eye height of the primary user.

References

[1] https://en.wikipedia.org/wiki/Illuminance

[2] CIE S 026:2018. CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light.

[3] CIE S 008/E:2001. Standard for Lighting of Indoor Work Places.

[4] GB 50034—2024. Standard for Lighting Design of Buildings.

[5] EN 12464-1:2021. Light and Lighting — Lighting of Work Places — Part 1: Indoor Work Places.

[6] ANSI/IES RP-3-20. Recommended Practice: Lighting Educational Facilities.

[7] GB 7793—2010. Hygienic Standard for Daylighting and Artificial Lighting in Classrooms of Primary and Secondary Schools.

[8] GB/T 9473—2022. Performance Requirements for Table Lamps for Paper Tasks.

[9] T/CIE 030—2020. Specification for Healthy Lighting Design in Primary and Secondary School Classrooms.

[10] DB31/T 539—2020. Specification for Lighting Design in Primary/Secondary School and Kindergarten Classrooms.

[11] T/SIEATA 000001—2020. Grading Evaluation of Classroom Lighting Quality in Primary and Secondary Schools.

[12] The WELL, 2019.

[13] GB/T 5700—2023. Methods of Measurement for Lighting.

[14] JJG 245—2005. Illuminance Meters.

[15] Cerqueira, D., Carvalho, F., and Bettencourt Melo, R. “Is It Smart to Use Smartphones to Measure Illuminance for Occupational Health and Safety Purposes?” Advances in Intelligent Systems and Computing, Issue 604, 2018, pp. 258–268.

[16] Jiang, J., Liu, D., Gu, J., and Süsstrunk, S. “What is the Space of Spectral Sensitivity Functions for Digital Color Cameras?” 2013 IEEE Workshop on Applications of Computer Vision (WACV), Tampa, FL, 2013, pp. 168–179.

[17] Vujica Herzog, Natasa, Buchmeister, Borut, and Spindler, Lea. “Comparison between Lux Meter Apps and Illumination Measuring Devices.” Chapter, January 2022. DOI: 10.2507/daaam.scibook.2022.03.