How to Measure Lights

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 it 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.

Comments

  1. Hi I know I may be clutching at straws but can anyone confirm what the lighting should be in an open office of IT staff with computers in front of them. We have an office with persistent offenders switching lights off. Although I have complained I’m advised there is no regs to state a minimum within an office in south Wales.

    1. Author

      Hi Barry,

      Check out our other article Lighting Levels in the Workplace it might help with the legal side of things. Unfortunately as advised, there is no strictly right answer to the correct lighting level, it is all about interpretation. Offices in general normally require a lighting level of minimum 100Lux with an average of 200Lux. This comes under ” Work requiring perception of details Offices, sheet metal work, bookbinding”. Depending on the type of work undertaken this could increase to a minimum of 200Lux with an average of 500 Lux ” Work requiring perception of fine details – Drawing offices, factories assembling electronic components, textile production”.

      Hope that helps

    2. Hi Barry,
      I work in front of a monitor as a CAD Technician.
      For me, the overhead lights create glare on my monitor. If I don’t resolve the glare, I get a headache.
      If I turn the overhead lights, the glare on the monitor goes away.
      Theresa

      1. Ask your employer to invest in a matte screen filter, or a hood. You can get filters that filter blue-light (This is what makes your eyes tired) and prevent reflection – this is what you need. You are potentially damaging your eyes by not have enough illumination.

  2. I am still not clear how to measure the LUX Level of LED Light. I mean one Bulb or one Tube Light or Track Light
    If i am measuring a LED Light in a dark room then the level is different and if i measure the Level with so many light brighten the room then it is higher. My Question is how i Get accurate Level of One Light by Lux meter.
    Please advise .

    1. Author

      Hi A.Ahmed,

      Light can be a little bit tricky.

      The light/bulb itself doesn’t have a lux level. It is the area that is being lit up that does. In your example, the second reading is higher because the light meter is picking up light from the other lights in the area.

      Is is almost impossible to measure the amount of light given off by 1 light in the conditions you describe.

      Ideally you want to use a lux meter to measure the amount of light hitting a workstation or similiar. So if the work station is below the light, hold the light sensor of your lux meter near the surface of the work station. (Where your eyes look). This will tell you how much light is in the working environment not how much light is produced by the nearest light bulb.

      Another reason your readings might be different is because normal light meters are not very good at measuring LED lights. Every LED bulb is slightly different and produces the light with a different variation of red blue and green light. To measure these accurately you need a light meter designed to measure LED lights. These work by applying a correction factor to compensate for the various types of LED light. Our own is linked here.

      Hope that helped

  3. Hi there,

    I am a little confused about which measurement will suit my purposes. I am running an experiment measuring pupil diameter using a remote eye tracking device mounted to a computer monitor. I am adjusting light via changes to computer screen luminance via adjusting the colour of the screen (e.g. white – grey – dark grey). There are going to be other lights in the room as well which will stay constant. My main goal is to measure how much light is hitting a person’s eye in a given condition. Is it most appropriate to measure candela/m2 or lux in this case? I have found so many conflicting points about the differences between luminance and illumination, regarding which is more appropriate for my purposes. Could you please indicate which is better/more appropriate and why?

    Any help is greatly appreciated. Thank you very much.

    1. Author

      Hi Jennifer,

      Sorry, we cannot advise on this very specific use of light meters as we would hate to provide inaccurate information that would affect your study.

    1. Author

      Hi Bharat,

      This varies dependant on your application. But most of the time the sensor should be placed on the surface that the person will be looking at as this will represent their working conditions. So if at a desk, place it on the surface of the desk etc.

  4. How can I check the light output of a car, I believe there must be regulations for minimum and maximum light output to an EU standard, but it seems MOT garages do not check for that, but only the direction.

    I am interested in finding this information from anyone that can help please.
    Thank you

  5. Why am i getting a reading of 0 lux in a room but things are still visable in that room?

    1. Author

      Hi Nathan,

      Three possible reasons I can think of,

      1. the sensor may have a protective cap you havn’t removed.
      2. Its a low light environment and your meter isn’t capable of measuring such low lux levels
      3. Its low light environment and your meter isn’t calibrated as is reading out of specification

      Hope Thats Helps

    2. Nathan,
      on a bright moonlit night, when the moon creates shadows you can see, the illumination level will be 0.2 – 0.5 lux. If your light meter doesn’t have a range adjustment that can detect that low, it will round it down to the nearest whole number – 0.

  6. Hello, if i am trying to measure Foot candles, does the measuring device have to be one foot from the fixture to get the correct “footcandle” reading. I don’t believe most devices will be correct if the distance more than the one foot. The closer i move the meter the more foot candles, the farther away the foot candles decrease. Thanks

    1. Author

      Hi Carl,

      Light diminishes as it travels by the square of the distance traveled. Your light meter whether reading in Foot Candles or Lux will tell you how much light is in the immediate area that the sensor is in.

      Footcandle meters measure how much light is in 1sq.ft around the sensor,
      Lux meters measure how much light is in 1sq.m around the sensor,

      You want the sensor to be placed at the point your eyes look at. So if working at a desk, put it on the surface of the desk. Holding your device 1 foot from the light source will only tell you how much light is in the area, 1 foot away from the light source. As you move further away from the light source, you should expect the reading to drop as the light diminishes. This does not mean the reading is incorrect, just that less light is reaching that point of the room.

    1. Author

      Hi Goke,

      I’m afraid we don’t offer specific usage advice as this comes down to individual health and safety laws/guidelines. The principles of using your light meter remain the same as in this article, but the levels required are down to your own interpretation.

  7. This was a great article at explaining light meters and lighting. I work for a Health Department and am always having to explain lighting, why it is important, why it changes when they purchase the “wrong” bulbs when one burns out, etc. to operators and my staff.

    I’m trying to find information on the different type of sensors and the best over-all/general purpose for multiple-light types. Any information of silicon photo-diode versus selenium photovoltac vs. color?

    1. Author

      Hi Debbie,

      I’m very sorry, but we don’t have any information related to that specific topic.

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