More about Illuminance

3 W LED flashlight

Overview

The Difference Between Illuminance, Luminance, and Brightness

Units

Light Meter

Illuminance in Work Safety

Illuminance in Photography and Videography

Low Light Cameras

Exposure Value

Relationship between EV and lighting conditions

Reflected light

Incident light

Illuminance in Museum Conservation

Illuminance in Other Fields

Overview

Illuminance is a measure of the amount of light that shines on a given unit of area of an object’s surface. This value depends on the human perception of brightness for light of different wavelengths, corresponding to different color. Illuminance calculations are adjusted for each wavelength because humans perceive light of a wavelength of about 550 nanometers (green) and close to it (yellow and orange) as brighter, while light of longer or shorter wavelengths (purple, blue, red) as darker. Illuminance is often likened with how bright an object appears to the human eye.

Illuminance is inversely proportional to the area over which the light is scattered. This means that for the same light source illuminance would be greater for a smaller area, and smaller for a greater area.

The Difference Between Illuminance, Luminance, and Brightness

LuminanceIlluminance

One can think of illuminance as the light that is going towards the object, or in particular the amount of light, just as it touches the surface. This is different from luminance, which is the amount of light reflected from the surface. It is easy to remember it if you think that:

Illuminance = ILluminance = Incident Light + luminance.

Luminance = light Leaving the surface. Leaving light

Our perception of brightness can be either the perception of the light reflected from the surface (luminance), or light going to the surface (illuminance), or even the total amount of light in the room. It could also be a combination of some or all of these, depending on the situation. We could also think of the area as being bright, or the object as being bright, or the light source as being bright. This is one of the reasons why the word "brightness" is not widely used in science.

Units

One of the most common units for measuring Illuminance is a lux. It is an SI unit. One lux is one lumen per square meter. A foot-candle is a non-metric unit for measuring illuminance. It is used in photography and film, as well as in some other areas. The word “foot” is used to refer not to body parts but to distance: a foot-candle is the illuminance of a one-candela light source per square foot, measured from one foot away.

Sverdlovsk-4 CdS precision light meter made in the USSR in the 1980s

Light Meter

A light meter is a device that measures illuminance by converting the light detected by the photodetector to electricity and then measuring it. There are light meters calibrated in different units, but lux meters are some of the most common ones. Light meters are widely used in photography and videography, in the area of occupational safety, in museum conservation, in plant cultivation, and in many other areas of human activity, where light plays an important role.

Illuminance in Work Safety

People who work in dim lighting conditions may suffer from deterioration of vision, depression, and other physiological and psychological problems. Therefore many health and safety codes for workplaces include specifications on what the minimum illuminance should be in the work environment. This is generally measured by the light meter. The meter adjusts the reading based on the area of the room to ensure that there is enough light for the entire room.

Illuminance in Photography and Videography

Video shooting

Cameras are equipped with devices for measuring illuminance, because a camera has to allow an adequate amount of light onto the film or the imaging sensor, based on the illuminance of the object being photographed. The illuminance in lux is then converted to determine the exposure necessary, taking into consideration the specifications of the camera and the photographer’s or videographer’s needs. Photographers and videographers also measure the illuminance to ensure that the light setup in the studio or on location is adequate. This can be done either by the camera or by an external light meter.

Still cameras can take pictures in low light conditions by opening the shutter for a long time, to allow enough light in. Video cameras are limited in this respect and need artificial lighting in dark conditions, otherwise the resulting video will not be bright enough. Some cameras work better in low light than others — camera manufacturers sell cameras designed for these conditions.

Low Light Cameras

Cell phone camera and lens

Low light cameras can either have superior optics or superior electronics. The optics allow more light in, and then work with the extra light. Electronics have less light but do a good job processing the light that is available. The superior electronics often produce more of the shortcomings described below compared to the cameras with superior optics. The drawbacks of powerful optics are that the camera lenses are large, heavy, and much more expensive.

Compact camera type 1/1.7” (7.60 x 5.70 mm) CCD sensor and lens

In addition a camera will have either one chip to process the light that is coming in, or three — one for each color (red, green, and blue). The latter type of camcorders is better because they scatter less light when breaking it down into three components by using a prism, compared to a filter used by the one-chip camera.

There are two different types of image sensors available — charge-coupled device (CCD) chips and complementary metal–oxide–semiconductor (CMOS) chips. In CCD chips а sensor for capturing light can sometimes be combined with аn image processor. CMOS chips can also combine the two together, although only in inexpensive cameras. In professional ones the sensor and the image processor are usually separate. CCD chips generally produce better quality video but CMOS chips are cheaper and less power-hungry.

A full-frame 24 x 36 mm CMOS sensor of a professional DSLR camera Canon 5D Mark II

The size of the chip also affects the quality of photographs and video — the bigger it is, the higher the quality of the image, and the smaller it is — the more noise it produces in low-light conditions. Larger sensors also require more powerful and heavy optics. They are used in more expensive cameras. It allows one to shoot professional quality video even with a still DSLR camera, such as a Canon 5D Mark II or Mark III, where the sensor is 24 x 36 mm in size.

The standard for how the light qualities of the camera are determined is not universal; it depends on the manufacturer. An attempt to create a universal standard was made by the Electronic Industries Alliance, but currently it is not universally accepted. This means that one camera that records in 2 lux may produce a very different quality of video from another camera with the same lux specifications.

Cameras that are marketed to work in low light conditions may not provide adequate quality, with problems including out of focus video and noise, such as grain and after-images for the bright areas. To address some of these issues the videographer can do the following:

If there is not enough light and the object is static, it is better to mount the camera on a tripod

• Mount the camera on a tripod;
• Use the manual mode;
• Avoid zooming in;
• Disable automatic gain (ISO) selection and autofocus;
• Set shutter speed to 1/30;
• Diffuse the lights;
• Use all the lighting already available in the environment, such as street lights and moonlight.

Despite the lack of standardization of information in lux sensitivity for different camcorders, it is still a good idea to get one with lux of 2 or less. It is important to note that even if the camera produces good quality video at low light, its lux value is the light that is coming towards the subject, but the camera actually catches the light that is reflected from it. The amount of light depends on the subject (some colors reflect light better than others) and on the location of the light. If the light is too far from the subject to illuminate it well, or if the subject is too far from the camera, then the quality would be diminished.

Exposure Value

Same picture with different exposure value

Exposure value (EV) is an integer representing several combinations of a shutter speed (or exposure time) and f-number in a photographic, movie or video camera. All combinations of shutter speeds and f-numbers that provide the same amount of light on a film or image sensor have the same EV value.

Several camera settings with the same EV value will provide the same exposure. However, the picture will not be the same because the f-number representing the aperture determines the depth of field, which will be different for different f-numbers, and the shutter speed representing the time when the film or sensor is exposed to light determines the amount of motion blur, which will also be different for different shutter speeds. For example, combinations of f/22 — 1/30 and f/2.8 — 1/2000 have the same EV, but the first picture will have increased depth of field and can be blurry, while the second picture will have shallow depth of field and will probably not have any blur.

Here the left picture is blurry because of longer shutter speed and the right picture is sharper because of the shorter shutter speed

Greater exposure values are used when more light is available. For example, exposure value (ISO 100) EV100 = 13 is appropriate for a landscape with clouds and EV100 = –4 is appropriate for shooting a bright aurora.

By definition,

EV = log₂ (N²/t)

or

2^EV = N²/t,   (1)

where
• N is the relative aperture (f-number, e.g. 2, 2.8, 4, 5.6, etc.)
• t is the exposure time in seconds (shutter speed, e.g. 30, 4, 2, 1, 1/2, ¼, 1/30, 1/100, etc.)

Depth of field at large (left) and small (right) aperture; note that both pictures were taken at the same exposure value

For example, for the combination f/2 and 1/30 the exposure value

EV = log₂(2²/(1/30)) = log₂(2² × 30) = 6.9 ≈ 7,

which can be used to shoot night scenes and window displays. The combination f/5.6 and shutter speed 1/250 gives

EV = log₂ (5.6²/(1/250)) = log₂ (5.6² × 250) = log₂ (7840) = 12.93 ≈ 13,

which can be used to shoot a landscape with clouds and no shadows.

Note that the argument to a logarithmic function is required to be dimensionless. The definition of the exposure value EV ignores the units of the denominator and uses only the numerical value of the exposure time in seconds.

The same exposure value 12 is set on a manual film camera (Зенит-ЕТ) and a digital camera (Canon 5D Mark II)

Relationship between EV and lighting conditions

Reflected light

Determining exposure by means of measuring light reflected from the object using a light meter

For light meters that measure reflected light, the camera settings (f-number and shutter speed) are related to subject luminance and ISO speed by the following incident light exposure equation:

N²/t = LS/K,   (2)

where

• N is the relative aperture or f-number
• t is the shutter speed or exposure time in seconds
• L is the average luminance of the scene measured in cd/m²
• S is the ISO speed (100, 200, 400, etc.)
• K is the reflected light meter calibration constant; Canon and Nikon use K = 12.5.

From (1) and (2), the exposure value is

EV = log₂ (LS/K)

or

2^EV = LS/K

Using K = 12.5 and ISO 100, we have the following equation for luminance:

2^EV = 100L/12.5 = 8L

L = 2^EV/8 = 2^EV/2³ = 2^EV–3.

This formula is used in the Luminance Converter to convert EV to cd/m² and vice versa.

Incident light

Determining exposure by measuring incident light using a light meter

For light meters that measure incident light, the camera settings (f-number and shutter speed) are related to subject illuminance and ISO speed by the following incident light exposure equation:

N²/t = ES/C,

where

• N is the relative aperture or f-number
• t is the shutter speed or exposure time in seconds
• E is the average illuminance of the scene measured in lux
• S is the ISO speed (100, 200, 400, etc.)
• C is the incident light meter calibration constant; C = 250 is commonly used.

Using C = 250 and ISO 100, we have the following equation for illuminance:

2^EV = ES/C = 100/250 E = 0.4 × E

E = 2.5 × 2^EV.

This formula is used in the illuminance converter to convert EV to lx and vice versa.

Note that though the Exposure Value vs Luminance and Illuminance Table suggests that it is possible to convert cd/m² directly to lux, this is not the case because lux is used to measure the amount of light that falls onto a surface while cd/m² is a unit to measure the perceived brightness of a surface. It all depends on the color and texture of the surface. For example, a surface covered with black velvet can receive a very high illuminance (incident light measured in lux) and it still has very low luminance. On the other hand, a shiny white car can be perceived as relatively bright even though the absolute amount of light that falls onto its surface can be even smaller than that on a surface covered with black velvet. That is why it would be very difficult to photograph a model in black velvet dress against a shiny white car.

Example of lighting conditions at which this EV can be used

More information about the exposure value.

Illuminance in Museum Conservation

Statue in the Château de Versailles, France

Illuminance and luminous intensity of the light determine the rate and the amount of damage that museum pieces incur. Measuring illuminance also helps conservation workers to ensure that people have an environment perceived as bright enough to appreciate viewing the pieces. Illuminance can be measured with light meters, but this may be difficult, because one has to be close to the artifact, which may require disabling the alarm and possibly other inconveniences. Therefore some conservationists prefer to use a camera as a light meter. It is generally used to determine whether lighting in the area is problematic and needs more testing, not for final testing.

A camera calculates the exposure based on the illuminance, so the illuminance value can be found by reverse calculating it. One can apply a formula to find it or use a table with the set of pre-determined values, but it is important to keep in mind that the camera absorbs some of the light and adjust calculations accordingly.

People who grow plants may use a light meter to ensure that the illuminance is adequate for each plant

Illuminance in Other Fields

In plant cultivation the amount of light in the environment is important because all plants need sunlight for photosynthesis, but each requires a different amount. Generally it is known how much light each plant needs, and people who grow plants may use a light meter to ensure that the illuminance is adequate for each plant.

References

This article was written by Kateryna Yuri