How to Calculate Illumination

What is Illuminance

Illumination intensity is a physical term that refers to the luminous flux of visible light received per unit area. Abbreviated as illuminance [1], unit Lux (Lux or lx). It is used to indicate the intensity of light and the amount of illumination of the surface area of ​​the object.

In photometry, “luminance” is the density of luminous intensity in a specified direction, but it is often misunderstood as illuminance. The international unit of luminosity is the candlelight received per square meter.

The light intensity has a great influence on the photosynthesis of organisms. It can be measured with an illuminance meter.

The illumination/illuminance on a surface illuminated by light is defined as the luminous flux illuminating a unit area.

Suppose the luminous flux on the surface element dS is dΦ, then the illuminance E on this surface element is: E=dΦ/dS.

1 lx=1 lm/㎡. When an object is uniformly illuminated by light, when the luminous flux obtained on an area of ​​1 square meter is 1 lumen, its illuminance is 1 lux. Lumen is a unit of luminous flux.

A point light source with a luminous intensity of 1 candela has a luminous flux of “1 lumen” per unit solid angle (1 steradian).

Candlelight (Candela), transliteration “Candela”. The concept of candlelight was first invented by the British, and it is a unit of luminous intensity.

At that time, the British used a pound of white wax to make a one-foot-long candle to define the unit of candlelight. But today’s definition has changed: heating with a black luminous body of one cubic centimeter until the luminous body melts into liquid, 1/60 of the amount of light emitted is the standard light source, and candlelight is the standard light source. The unit of light emitted.

How to calculate illuminance

Illumination calculation method: use coefficient method to calculate average illuminance–

Average illuminance (Eav) = total luminous flux of light source (N*Ф) * utilization factor (CU) * maintenance factor (MF) / area area (m2) (applicable to indoor or stadium lighting calculation)

Utilization factor: 0.4 for general indoor, 0.3 for sports

Maintenance factor: generally 0.7～0.8

• Example 1: Indoor lighting: 4×5m room, using 9 sets of 3×36W grid lights

Average illuminance=total luminous flux of light source×CU×MF/area

＝(2500×3×9)×0.4×0.8÷4÷5

＝1080Lux

Conclusion: The average illuminance is above 1000 Lux

• Example 2: Stadium lighting: 20×40m venue,

Use LedsMaster 1000W LED floodlight 60 sets

Average illuminance=total luminous flux of light source×CU×MF/area

＝(130000×60)×0.3×0.8÷20÷40=2340 Lux

Conclusion: The average horizontal illuminance is above 2000 Lux

• A design case of the average illuminance of a football field:

Design conditions: Standard football field is 105 meters long, 68 meters wide, light poles are 18 meters high, and the distance between light poles is 36 meters.

The utilization factor is 0.7, the maintenance factor is 0.8, the number of lamps is 36 sets,

What is the average illuminance in the football field?

Lamp solution: The lamp adopts LedsMaster 1000W anti-glare floodlight, luminous flux 170,000Lm, color temperature 5600K, color rendering above Ra90.

According to the formula:

Eav=(36 sets X 170000 Lm X 0.7X0.8)/(105m X 68m) =110880.00÷196.56m2=480 Lux

Remarks: The lighting design must require an accurate utilization factor, otherwise there will be great deviations.

The main factors affecting the utilization factor are as follows:

*Light distribution curve of lamps

*Light output ratio of lamps

*Reflectivity in the venue, such as lawns, walls, stands, etc.

*Floodlight beam angle

Related terms

• 1. Natural lighting and artificial lighting

Sunlight is natural lighting, and light lighting is artificial lighting.

• 2. Light cycle and light time

In nature, 24 hours a day and night is a light cycle. The time with light is the bright period, and the time without light is the dark period. In natural light, the sunlight time (bright period) is generally calculated as the sunshine time; in artificial light, the light exposure time is the light time, and the 24h light period is the natural light period; the longer or shorter than 24h is called unnatural light Cycle; if there is only one bright period and one dark period within 24h, it is called single-period illumination; if there are two or more bright or dark periods within 24h, it is intermittent illumination. The sum of the bright period in a photoperiod is the photoperiod.

• 3. Luminous intensity

The amount of luminous flux transmitted by a light source within a solid angle in a certain direction. Unit: Candela (candela, cd).

• 4. Luminous flux

The light energy radiated by the light source per unit time is called the luminous flux of the light source, and its unit is lumens (the amount of light on a 1 square foot area that is 1 foot away from the 1 candle light source is 1 lumens).

Under direct sunlight in summer, the light intensity can reach 60,000 to 100,000 lx, outdoor 10,000 to 10,000 lx without sun, 100 to 550 lx indoor in bright summer, and 0.2lx at night under the full moon.

Incandescent lamps can emit approximately 12.56 lx of light per watt, but the value varies with the size of the bulb. Small bulbs can emit more lumens and large bulbs are less. The luminous efficiency of fluorescent lamps is 3 to 4 times that of incandescent lamps, and the lifetime is 9 times that of incandescent lamps, but the price is higher. Of the light emitted by an incandescent bulb without a lampshade, about 30% of the lumens are absorbed by walls, ceilings, equipment, etc.; the poor quality and darkness of the bulb will reduce many lumens, so only about 50% of the lumens can be used.

Generally, when there is a lampshade and the lamp height is 2.0～2.4m (the distance between the bulbs is 1.5 times the height), 1W bulbs per 0.37㎡ area or 2.7W bulbs on 1㎡ area can provide 10.76 lx. The height of the bulb installation and the presence or absence of the lampshade have a great influence on the light intensity.

LED light distribution curve

  Definition of light distribution curve:

It refers to the light intensity distribution of light sources (or lamps) in all directions in space.

The curve formed by marking the light intensity values ​​of each position on the polar coordinate graph is the light distribution curve of the lamp.

  How to express light distribution curve

There are generally three ways to express the light distribution curve: one is the polar coordinate method, the other is the rectangular coordinate method, and the third is the equal light intensity curve.

a. Polar coordinate light distribution curve:

On the metering plane passing through the center of the light source, the light intensity values ​​of the lamps at different angles are measured. Starting from a certain direction, the light intensity of each angle is marked with a vector using the angle as a function. The connection at the top of the vector is the polar coordinate light distribution curve of the lighting fixture. If the luminaire has a rotationally symmetrical axis, only the light intensity distribution curve on a photometric surface passing through the axis can be used to illustrate the spatial distribution of its light intensity.

If the light distribution of the luminaire in space is asymmetric, the light intensity distribution curves of several photometric planes are needed to explain the spatial distribution of its light intensity.

b. Rectangular coordinate light distribution curve:

For concentrating lamps, because the beam is concentrated in a very narrow solid angle, it is difficult to express the spatial distribution of its light intensity in polar coordinates, so the right-angle light distribution curve representation method is used, and the vertical axis represents the light intensity map. I. Use the horizontal axis to indicate the projection angle of the beam. If it is a luminaire with a symmetrical axis of rotation, only one light distribution curve is needed to represent it, and if it is an asymmetrical luminaire, multiple light distribution curves are required.

c. Light intensity curve:

The curve connecting the tops of the vectors with equal light intensity is called the equal intensity curve, and the values ​​of the adjacent light intensity curves are arranged in a certain ratio, and the graph composed of a series of equal intensity curves is called equal intensity curve. Graphs, commonly used graphs include circular graphs, rectangular graphs and positive arc graphs. Since the rectangular network diagram can explain both the light intensity distribution of the lamps and the regional distribution of the light quantity, the iso-intensity curve diagrams of the floodlight lamps are all rectangular network diagrams, which we will not introduce here.

Light output ratio

Light output ratio, i.e., lamp efficiency, refers to the ratio of the luminous flux value emitted by the lamp measured under specified conditions to the sum of the measured luminous flux values emitted by all light sources in the lamp. There are many classification methods for lighting fixtures, such as classification according to purpose, classification according to the luminous flux distribution ratio recommended by CIE, and classification according to dustproof, moistureproof, and electric shock resistance.

  Main classification

According to the classification of lamps recommended by the International Commission of Illumination (CIE) (indoor lighting)

According to the recommendations of the International Commission on Illumination (CIE), luminaires are divided into five categories according to the proportion of luminous flux in the upper and lower spaces: direct type, semi-direct type, fully diffused type (including direct-indirect type with little horizontal light), and semi-direct type. Indirect and indirect.

  (1) Direct lighting luminaire

Most of the luminous flux (90-100%) of this type of lamps is directly illuminated below, so the luminous flux of the lamps has the highest utilization rate.

  (2) Semi-direct lighting luminaire

Most of the luminous flux (60-90%) of this type of luminaire shoots into the downward hemisphere space, and a small part shoots upwards. The upward component will reduce the hardness of the shadow produced by the lighting environment and improve the brightness ratio of each surface.

(3) Diffuse or direct-indirect lighting (diffused lighting luminaire)

The upward and downward luminous fluxes of the lamps are almost the same (40%-60% each).

The most common is the opalescent glass spherical lampshade, and other closed lampshades with diffuse and transparent shapes have similar light distribution. This kind of luminaire throws light evenly in all directions, so the luminous flux utilization rate is low.

(4) Semi-indirect lighting luminaire

The downward luminous flux of lamps accounts for 10%-40%, and its downward component is often only used to produce brightness commensurate with the ceiling. Too much of this component or improper distribution can also cause some defects such as direct or indirect glare.

The translucent cover that is open above falls into this category. They are mainly used as architectural decoration lighting. Since most of the light is directed to the ceiling and upper wall, the indirect light in the room is increased, and the light is softer and more pleasant.

  (5) Indirect lighting luminaire)

A small part of the luminous flux (below 10%) of the lamp is downward. When the design is good, the entire ceiling becomes a lighting source, achieving a soft and shadowless lighting effect. Because the lamps have very little downward light flux, as long as the layout is reasonable, the direct glare and the reflected glare are very small. The luminous flux utilization of this kind of lamps is lower than the previous four.