Luminance and Illuminance

The terms of ‘Luminance’ and ‘Illuminance’ appear, at first glance to have the same or at the very least a similar meaning, however nothing could be further from the truth. When using these terms, they are often used interchangeably, yet they have very different meanings and using one or both terms incorrectly could cause a misunderstanding with a specification, or at the very least, some confusion.

It is very much hoped that during this editorial, the content will distinguish the difference between the two terms of luminance and illuminance, and in attempting to achieve this, there will be a need to be a consideration of another important term, that being ‘luminous flux’. From a previous article on the changes in quantifying lamps based upon the measurement of power consumption to that of performance related metric, where the lamp is termed by the amount of ‘luminous flux’, which in simplistic terms, is the amount of visible light being emitted by the lamp.

The International Commission on Illumination, known as the CIE, which takes its name from its French title, the ‘Commission Internationale de l´Eclairage’ has published CIE-S-017:2020 ILV, the International Lighting Vocabulary to promote the standardisation in the use of terminology related to the science and art of light and lighting, with the two terms of luminance and illuminance being given below.

The term ‘Luminance’ (17-21-050) is the ‘density of luminous intensity with respect to projected area in a specified direction at a specified point on a real or imaginary surface’, and is given by the symbol ‘L’

The term ‘Illuminance’ (17-21-060) is the ‘density of incident luminous flux with respect to area at a point on a real or imaginary surface’ and is given by the symbol ‘E’.

Having now seen the definitions given by the CIE, let’s look at the two terms separately, in a little more detail.

Luminance relates to the amount of light emitted, which either passes through, or is reflected from a surface and is considered as being the apparent brightness of an object, or how bright an object appears visually. Luminance is often used interchangeably with the word brightness, however this is not deemed as being completely correct, as luminance has a quantifiable value which can be measured, whereas brightness has only a perceived value, being that of a visual sensation, which differs for different people when viewing the same object. Luminance is measured as being luminance intensity, which is a measure of the brightness of a beam of light in a particular direction, and for the UK and Europe, the unit used to express luminance is the ‘Candela’, with the measurement being given in candelas per square metre (cd/m2).

As is has been stated that luminance has a value, and is something which can be seen, and there being possible natural extremes for the value for luminance, where the sun will have a particularly high value of luminance, and the moon will have a very low value of luminance.

Illuminance relates to the amount of light which actually impacts on a surface, such as a desk located within an office, a work bench within a factory or the floor. This illuminance is often referred to as being the ‘incident light’, which is when a beam of light falls on a surface, thereby illuminating a given area. The value of illuminance is based upon the amount of light per given area, and for the UK and Europe, the unit used to express illuminance is ‘Lux’ with the measurement being given in lumens per square metre (lm/m2).

For the design of lighting within project applications, the calculated value of illuminance is a somewhat complex topic and whilst there is other criteria determining the value of illuminance, it will mainly depend on both the lumen value of the light sources installed within the space and the distance between those light sources and that of the surface being illuminated. Basically, the closer the light sources are to the surface being illuminated, the higher the value of illuminance for the same intensity of the source, however it is also known that the area being illuminated by a single light source will reduce in size as it gets closer to the surface being illuminated.

There are a couple other terms worthy of consideration for illuminance, that of ‘horizontal illuminance’, where the surface being illuminated by the light source is horizontally placed, being generally parallel to the floor, and also ‘vertical illuminance’ where the surface being illuminated is vertically placed, usually being a wall or other surface, either internally or externally.

For lighting projects, the values of required illuminance vary greatly and will depend upon the visual task being carried out, where for a corridor, the value will be low and for an area where precision work is being carried out, the value will be high, with the average commercial office, being somewhere in the middle.

In concluding, luminance can be seen by the human eye, its value will vary according to the ability of the surface to reflect the light, and is a measure of the light being reflected, being dependent upon the type of surface upon which the light falls on and which is also dependent upon the quantity and light distribution pattern of the light sources. Whereas, illuminance is not seen by the human eye, its value will vary depending upon the distance between the light source and the surface being illuminated, and it measures the light which is illuminating the surface, that being totally independent of the construction and make-up of the surface as well as being independent of the quantity, light distribution pattern or the direction of the installed light sources.