The Light Meter An Introduction to Photometry

What is Photometry Photometry focuses on assigning numerical values to light energy emitted from wavelengths between 400nm and 700nm. This is the visible light portion. A subject called Radiometry focuses of most of the EM spectrum (400nm to 1000nm). This light meter from Quantum Instruments is just for Photometry use. Think of this value as what Amps or Volts is to electricity.

The Light Meter All light meters holds a plain silicon detector inside of it (Silicon Photo Diode) which can be used for radiometry or photometry. This meter can only be used for photometry however due to the green filter which only allows wavelengths from 400nm to 700nm to arrive to the detector. This filter simulates the energy or light our eyes can see and assigns numerical values for them. It is called a light meter, Photo Meter, and sometimes Photometer

What Exactly Does it Measure? This specific light meter can measure two variables. One is the Illuminace value which is measured in Lux. This white filter must be placed over the green filter. A lux is equal to [lm/m^2]. The other variable is the Luminance value which is measured in [cd/m^2]. To measure this the aperture must be placed over the green filter.

Illuminance Illuminance measures the intensity or power of the light over a surface, per unit area. [lm/m^2] or [Lux] This White filter is used to collect light from all angles and get rid of the dimensional aspects of where the light is coming from. To capture the illuminance value it must integrate all the light into one source.

Luminance Luminance is the measure of the intensity of the light per unit area of light traveling in a certain direction. This direction of the light is the difference between Illuminance and Luminance The aperture is used over the green filter to measure Luminance because it only allows light in from a fixed, known angle. Luminance is measured in [cd/m^2]

How do you use it? The Side which Says Photometer should be facing up when held. The Side which Says Photometer should be facing up when held. The button on the right is called Measure control. You hold this button down to take a measurement. If the Display shows three dashed lines, the light intensity is out of range. If the light intensity is out of range you hold on Range button (on the left side). This will change the decimal places. Photometers need to be calibrated to work correctly.