Illuminance

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: the incident luminous flux per unit area on a surface

Categories: article belongs to category light detection and characterization light detection and characterization, article belongs to category optical metrology optical metrology

Units: lux (lx)

Formula symbol: ($E_\textrm{v}$)

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DOI: 10.61835/nzj Cite the article: BibTex plain text HTML Link to this page! LinkedIn

irradiance — Figure 1: Illuminance is received luminous flux per unit area.

The illuminance is a quantity defined in the area of photometry, which is used for quantifying the intensity of illumination, e.g. on an office desk. In contrast to a physical quantity like the irradiance (the corresponding quantity in radiometry), it takes into account the wavelength-dependent sensitivity of the human eye, typically for photopic vision. The related radiometric quantity is the irradiance, specified in units of W/m², from which one can calculate the illuminance using a luminosity function.

The SI units of that quantity are lux (lx). 1 lux corresponds to a luminous flux of 1 lumen per square meter (1 lm/m²). This is what is approximately reached by the light of the full Moon on the surface of Earth.

Simple Examples

As a simple example, if you illuminate a certain area ($A$) homogeneously with a certain total luminous flux ($\Phi_{\rm v}$), the area receives an illuminance ($E_{\rm v} = \Phi_{\rm v} / A$).

If you have a small light source with a certain luminous flux ($\Phi_{\rm v}$), which radiates isotropically (i.e., with equal intensity in all directions), in a distance ($d$) from that light source you get an illuminance ($E_{\rm v} = \Phi_{\rm v} / (4\pi d^2)$).

If you use an LED lamp with a certain luminous flux, you will generally get a higher illuminance in forward direction than based on the equation above, as the radiation is more or less concentrated in some direction.

Measurement of Illuminance

multipurpose instrument with photometer function — Figure 2: A multipurpose instrument which also serves as a photometer (luxmeter), displaying the illuminance on a desk.

Illuminance values can be measured with photometers. Such a device can contain a photodiode, equipped with a suitable optical filter so as to match the wavelength-dependent responsivity of the human eye.

Figure 2 shows a simple multipurpose measurement device with a built-in photodetector, which is sufficient e.g. for checking the illuminance at a workplace. Dedicated photometers (luxmeters) often have a separate measurement head, which is attached to the display instrument with a cable.

For accurate illuminance measurements, the used photodetector should have the correct angular dependence of its sensitivity. This means that incident photons should contribute to the measurement signal with no dependence on their incidence angle.

Note that due to that accommodation of the eye to different light levels (see below), it is very hard to reliably estimate illuminance levels without using a measurement instrument.

Recommended Illuminance Levels in Rooms

Illuminance values are useful in the context of the illumination of rooms. For office work and reading paper documents, it is recommended to have an illuminance around 500 lx on the desk. For comparison, full sunlight incident on a white paper generates an illuminance of the order of 100,000 lux, which can be uncomfortable. On cloudy days, the illuminance can easily drop to 1000 lx or even below.

Generally, artificial illumination in rooms produces illuminance values which are far below those in direct sunlight, and which are nevertheless sufficient for perfect vision in most cases. Exposure of eyes to high illumination levels, e.g. to bright sunlight, accelerates the aging of the retina (mostly its central part, the macula) and the eye's lens – the latter mainly due to the ultraviolet content of sunlight, while blue light components are also known to lead to macular degeneration over longer periods of time. Therefore, it is recommended to use effective eye protection under intense light conditions.

Note that the human eye can adapt to different illumination levels in a wide range: We have at least some scotopic vision (without color perception) at a very low illumination levels, even below 10⁻⁴ lx, while substantially higher illuminance levels than those of sunlight can be tolerated for a while. Partly, that adaptation is achieved by opening or closing the iris, and partly by using different light sensors: three different kinds of cones (for photopic vision with color perception) and one kind of rods (for scotopic vision with substantially higher sensitivity but no color perceptions).

Illuminance vs. Luminance

Illuminance must be strictly distinguished from the similar term luminance. These two quantities are very different, and they even have different units (lm m⁻² vs. lm sr⁻¹ m⁻²). Illuminance characterizes light inpinging an illuminated object, while the luminance is relevant for the visual brightness of an object and also takes into account the angular distribution.

More to Learn

Questions and Comments from Users

2020-07-31

How to measure illuminance with an irradiance meter for the range 300–900 nm?

The author's answer:

The described instrument, as I understand it, can be used for measuring the irradiance in the given spectral range. This will not be sufficient to measure the illuminance caused by radiation in that range, since the wavelength-dependent sensitivity of the eye cannot be taken into account.

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