How to measure light | Using your light meter correctly

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; this article offers a basic introduction to what light is and how it is measured along with a guide on how to use a Light Meter (Luxmeter).

What is 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.

Types of 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

How to Measure Light

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

An alternative measurement of light is the foot-candle, this works the same way as lux except 1 foot-candle is 1 lumen per square foot.

Instruments for Measuring Light

The simplest way to measure light is to buy a light meter/ lux meter, the two phrases are often interchangeable. Light meters contain a sensor that converts the light energy into an electrical charge that can give the user a reading. They are typically small enough to be hand-held and easily carried about.

You can view our range of light meters here.

Using the light meter is simple. After taking the cap off the sensor, simply place it on a surface where a task is carried out such as the centre of a desk. It is important the sensor is placed on the surface as this is where the light is reflected into the user eye and represents the true level of light they receive. Holding the light meter above the surface would produce potentially inaccurate readings. The lux reading should then be displayed on the display.

When to use a Lux Meter

(If you skipped it, you may want to read section 1 of this guide) There are a few things you should be aware of when using a lux meter. This is mainly to do with the fact that different wavelengths of light are not sensed equally by the human eye. If all wavelengths contained the same intensity of light, the lux reading would be the same but the human user may see more light of a certain colour and the light could appear brighter.

To better correlate lux meters to the human perception of light, they are configured to the CIE standard illuminant A. This adjusts the light meter to expect light distributed over wavelengths of a domestic tungsten-filament light.

Because of these adjustments, a standard lux or light meter is ideal for use in areas that utilise incandescent lighting. They can also be used for fluorescent lighting areas but are subject to a small error in measurement. This makes them ideal tools for checking lighting in most workplace environments.

If your workplace is filled with LED lighting, you may need a different solution.

LED Light Meter

With the ever growing success of LED lighting a need has arisen for specialised LED Light Meters. LED lights produce white light very differently to incandescent lights. A traditional lux meter may give an accurate reading of 500 lux, but the human eye can’t visibly see all 500 lux and may actually only be able to see 300 lux. Ultimately this result in inaccuracies. You can get around this by using a specialised LED light meter.

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