What is a goniophotometer
Written by: Vinnie van Rooij
The Goniophotometer is a device that can measure the properties of light that is emitted by a light source. This gives insight in the uniformaty of the emitted light, the direction and the properties of light at certain angles and distances.
Contents
What is a Goniophotometer
When a light source is active, it emits a light. This light has a certain color, a certain “strength” and can differ at certain angles. These properties are divided by wavelength, fase, frequency, amplitude etc. The goniophotometer is able to measure the amount of luminous flux that is emitted by a light source and the amount of luminous intensity distribution. Some devices can also determine the distribution of color temperature and the color uniformity.
The goniophotometer has become very popular with the increasing developments of LED lighting. LED has the tendency to direct light into a certain direction, which is called a Lambertian souce. A regular lightbulb spreads light out in an almost homogeneous radial distribution, sending out light in all directions almost at the same intensity.
The Goniophotometer is withint the automotive industry used to determine the type of light from a headlight, in order to comply with the regulations.
Luminous flux
The luminous flux is the total amount of emitted light from a light source. Regardless of direction. For example: a laser gives a very high amount of luminous flux in 1 small spot, but hardly to no luminous flux at all in any other direction. An old lightbulb is very different, in the sense that it sends out the same amount of luminous flux in any and all directions. The total light output from both sources can still be identical. The laser focusses it in 1 spot, where the old lightbulb spreads it out over a larger area.
Luminous intensity
The luminous intensity is the amount of light emitted by a lightsource from a certain angle and from a certain distance. A directed light can give a very high amount of luminous intensity from a certain angle, and almost none from a slightly different angle. In this case the above mentioned laser would have a high luminous intensity in 1 small spot, but hardly to no luminous intesity in any other direction. The old lightbulb would have a very low luminous intensity, but it would be equal in all directions.
Distribution of color temperature
The distribution of color temperature is based on certain types of light. These range from 1000 kelvin (fairly red-ish) to 27000 kelvin (very blue-ish). For reference, 2500 to 5000 kelvin is often considered “warm white”, while 5000 to 7500 kelvin is considered “cold white”.
Color uniformity
When light is measured from different angles and different distances, the properties of the light can change. A high color uniformity means that the color temperature is evenly spread throughout different angles, whereas a low color uniformity means that the color temperature differs greatly with different angles.
Types of Goniophotometer
There are generally 3 types of Goniophotometers:
Type A
Only measures from a fixed horizontal axis.
Type B
Measures from a fixed vertical axis.
Type C
Measures both vertical and horizontal axis.
Differences between a Goniophotometer and a Glossmeter
The glossmeter is a device that measures the amount of light that is reflected by a surface, whereas the goniophotometer measures the properties of the emitted light. The Goniophotometer was probably used to develop the lightsource that is used in the glossmeter, because this light nees to emit a uniform glow of light with very steady charateristics.
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