Measuring light has become a common practice in various walks of life, from making sure your employee’s are operating in safe working conditions to checking lighting levels for photography or set design. Measuring light comes with a few considerations.

What is light
How to measure light
Instruments for Measuring Light

Feel free to skip this section, as it isn’t vital to understanding how to measure light correctly but is useful supporting information.
Let’s try to keep this simple. Light is a form of electromagnetic energy that travels in waves. These waves have both a wavelength and a frequency. Human beings have receptors that can sense particular wavelength’s and process it into images. These wavelengths exist between 400 and 700nm. Individual colours are present around certain wavelengths. See below…

Blue 420nm
Green 525nm
Red 635nm
You may have heard the terms infrared and ultraviolet. Infrared is when the wavelength are longer that what we can see and ultraviolet when they are shorter. Both have uses in different types of technology. When energy is emitted across all 3 of these wavelengths at once we get what we know as white light.

As a general rule your workplace will have 1 of three types of light fitting, these are light sources based on heat know as incandescent, Fluorescent lights and LED’s. Each one of these produces light in a different manner.

Incandescent light is radiated electromagnetic energy that is emitted across all wavelengths, when we see all wavelengths things appear white. Different temperatures will alter the amount that each wavelength is used.
Fluorescent Lights are technically outside of our visible range, they are ultraviolet and below 400nm. However, an interaction with a coating on the inside of their tubes makes is visible white light that we can use.
LED lights are a little more complex and achieve white light through a mixture of red, green and blue LEDS or methods similar to Fluorescents

The easiest way to understand how light is measured is by picturing the typical light bulb with a filament that heats up producing light (Incandescent if you read the previous section). The filament is the source of the light and is at the centre of a sphere with light being emitted in every direction. The total amount of energy of all the light produced is known as the “luminous flux”.

You are probably familiar with the Lumen; this is the measure of light intensity people have generally heard of. The base unit of luminous intensity is the candela, (a single lit candle gives off roughly 1 candela). One candela per steradian (an area in a cone shape starting from the source of light) is known as a lumen.

When we measure light, we are interested in how many Lumens fall on a surface; this is what we know as lux. One lux is one lumen per square meter.

A working example, We have a light source whose total light produced (luminous flux) is 1000 lumens. If we could focus this onto the surface of 1 square metre we would have an illuminance of 1000 lux. However if the same light was spread out over 10 square meters, we would only have an illuminance of 100 lux.

Examples of light levels
Very Bright Summer Day…………….100,000 Lux
Full Daylight………………………………10,000 Lux
Overcast Summer Day………………..1,000 Lux
Very Dark Day……………………………100 Lux
Twilight……………………………………..10 Lux
Full Moon < 1 Lux