Unique human visual perception presents challenges in light measurement
The visible light portion of the electromagnetic spectrum, sometimes called the optical spectrum, encompasses wavelengths approximately 400 to 700 nm containing most of the colors discernible to the human eye. However, the human eye is not equally sensitive to all wavelengths of light, being most sensitive to green at 555 nm under normal lighting conditions then to other wavelengths.
Complicating attempts to measure human light perception is the individual variation in our ability to distinguish, describe, and reproduce color. A summary of average human visual perception of light would be required if any method for measuring it or reproducing its color was to be developed.
The CIE or Commission internationale de l'clairage (International Commission on Illumination) is the international authority on matters involving light, illumination, and color. In an attempt to solve the problem of human color perception, the CIE in 1931 developed one of the very first mathematically defined color spaces, the CIE XYZ color space.
The human eye contains receptors for short, middle, and long wavelengths, also referred to as blue, green, and red respectively, and therefore a color sensation would have to be defined as three parameters. A specific method for assigning three numbers, also known as tristimulus values, to represent each color, is called a color space.
Based on direct human eye measurement experiments performed in the 1920's, an RGB color matching model of human vision was developed, and from this the CIE XYZ color space was derived. The central color matching function in calculating the CIE XYZ color space was the photopic luminosity function which represents the typical human eye response under normal lighting conditions and is referred to as the standard observer.
Scotopic luminosity represents the typical human eye response in dark conditions where no color perception exists, and mesopic luminosity is a combination of the photopic and scotopic functions where minimal color perception exists and represents the human eye response in dimly lit conditions.
Having developed a basic color space based on a standard observer, we can then quantify the perceptual brightness of a light by direct measurement with devices that have the same response to light as that of the standard observer in a science now called photometry. Photometric light measurement is widely used in many industries to specify, monitor, and control the brightness of task, street, safety, automotive, and aerospace lighting to name a few.
It is important to note that the CIE XYZ color space can only describe colors of light itself and not objects, since the perceived color of an object will depend on the surface and color properties of the object itself, that of nearby objects, and the ambient and direct lighting properties, but it does serve as the foundation for colorimetry, the science of color measurement or reproduction.
Many additional color models, measurement techniques and instruments have been developed subsequent to this groundbreaking research to objectively reproduce color in a wide variety of analog and digital media.
International Light Technologies covers the gamut of photometric measurement
ILT offers photometric measurement instruments that are unmatched in accuracy, sensitivity, and versatility for photopic and scotopic applications including illuminance and luminance, as well as extensive color analysis via the powerful feature set of either the ILT950 or ILT550 spectroradiometer.
Use the table below to identify the system (meter + detector) that meets your specific application. Use the table to find the spectral range you wish to measure. The table can be filtered to show our meters by type, (e.g., hand-held), as well as searching on the minimum and maximum spectral range you wish to measure. The tables can also be sorted to group systems by meter type, spectral range, measurement range, and units. Click the product link of the system to view it's details.
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* All Radiometers/Photometers/Spectroradiometers are NIST Traceable.
* If units of measure are not shown please contact us (empirical units also available i.e. fc, fL, nits, lm/ft²).