10 Key Definitions You Need To Know When Purchasing a Light Meter

Comparing the specifications of different light meters can be an overwhelming experience. If you have ever felt bamboozled by all the scientific jargon, don’t worry, you’re not alone. To ease your pain, we asked scientists at Instrument Choice to define (in simple terms) the key definitions you should know to select and accurately operate your light meter. We’ve also thrown in some helpful tips.

10 Key Light Meter Definitions

Comment: Some of the definitions below have quaint origins. You’ll see what we mean when you read on.

Definition #1: Lumen – This is a unit of total visible light that is emitted by a light source. The term encompasses light that’s radiated in all directions from its source.

Definition #2: Lux – This is a unit of light intensity.It is equal to one lumen (see definition #1 above) per square meter.

Definition #3: Candela – A Candela unit is a unit of luminous intensity. A point of reference might help here - a typical wax candle produces light with a luminous intensity of roughly one Candela.

Definition #4: Footcandle - A footcandle is a unit of light intensity. One footcandle is the illuminance cast on a surface by a one-candela source that is one foot away. (Lux measurement is the term used where the metric system is in use. Footcandle is the term used where the imperial measurement system is in use.

Definition #5: Luminous Flux - This is the measure of the perceived power of light.More specifically, it is energy radiated over wavelengths that are visible to the human eye.

Definition #6: Spectral Sensitivity – This refers to the relative efficiency of a device to detect light. The reference for the spectral sensitivity scale in the average capacity of the human eye. The agreed maximum spectral sensitivity of the human eye under daylight conditions is at a wavelength of 555 nanometres (nm), while at night, the peak shifts to 507 nm [Note: A nanometre is equal to one billionth (short scale) of a metre (0.000000001 m]. Figure 1 (below) shows the spectral sensitivity peak at 555nm.

Definition #7: Luminosity Function/Luminosity Efficiency Function – This describes the spectral sensitivity of average human visual perception of brightness. Different luminosity functions apply under different lighting conditions. Some specifications refer to the luminosity function as the “photopic luminosity function.”

Definition #8: CIE Photopic Spectrum/Curve - Humans perceive light through a spectrum of colours from long-wavelength reds to short-wavelength purple. The CIE photopic luminosity function (y(λ) or V(λ)) is a standard function established by the Commission Internationale de l'Éclairage (CIE). This function is used to convert radiant energy into luminous visible energy (light). Figure 1 below depicts the CIE Photopic Spectrum.

Figure 1 The CIE Photopic Curve with the daylight spectral sensitivity peak around 555nm

Definition #9: Cosine Accuracy/Directional Response – Directional Response refers to how a meter detects light that lands on a metering sensor at different angles. Cosine accuracy or correction refers to the human perception of light incident on a surface. What does this mean? Well, light from above is perceived by a human to be brighter than light emitted at lower angles. Because professionals use light meters for health and safety applications, these devices can have cosine correction built into their processors to correct results based on the angle of light collection.

Photodiode Definition #10: - A photodiode is a semiconductor that converts the incoming light to electrical current. A light meter’s sensor conducts electrical current directly proportional to the amount of light that it measures. This technology is applied to convert the energy collected by your light meter into a numerical reading.

Phew, that’s the top 10! We hope that by referring back to this list, you can quickly gain the understanding you need to make informed decisions about light meters.

Now here’s a bonus we’ve thrown in to help you better select the right light meter for your job.

Five Tips to Consider When Purchasing Your Light Meter

Tip #1: Accuracy: Depending on your desired use, you may need a more or less accurate meter. A meter used in professional testing or for verification purposes will need to have a higher degree of accuracy compared to a meter that is for home or casual use.

Tip# 2: Lens cover: A lens cover will prolong the life and the accuracy of your light meter by protecting the sensor. If you have one, use it.

Tip #3: Intended use: Not all light meters are created equal. Check that the light meter you are using or wish to use is suitable for your application. For example, not all light meters can measure LED lighting, and you will need a specific meter for this job. And don’t overspend on features you don’t need for your application, but ensure the meter does everything you need it to do. If you are not quite sure, feel free to contact us. We’ll help you find the perfect meter at the pricepoint you require.

Tip #4: Additional features: If your budget allows, there are many more sophisticated light meters available, some with data logging and data download capabilities.

Tip #5: Cosine Correction: Depending on the intended application, you may need a device with cosine correction to adjust for the angle that your meter collects light to ensure more accurate results.

Conclusion

Although light meters are commonplace and easy to use devices, the science and terms behind light meters can be challenging. Understanding of the ten terms and five tips listed above will help you better choose a light meter and take more precise measurements.

If you need further assistance with any of the above terms or need a hand finding the perfect light meter to suit your needs, contact an Instrument Choice scientist. We’re here to help!

Call 1300 737 871 or email [email protected].