A useful light ball

A scientist setting up a lamp for testing in the 2m integrating sphere

The integrating sphere is used to quickly measure the total light output (luminous flux) of a lamp. It is used for a variety of optical, photometric or radiometric measurements.

The lamp being tested is placed in the center of the integrating sphere. At one side of the sphere is a light meter which measures the light output of the lamp. Between the lamp and the light meter there is a baffle to prevent the meter from seeing any direct light from the lamp.

The inside of the sphere (including the baffle) is coated with a very white paint that reflects all wavelengths equally. This allows to get very accurate measurements. The light from the lamp bounces around the sphere until it reaches the light meter.

Measuring light features

The CIE chromaticity diagram

There are many optical instruments used to analyze the properties of light or optical materials.

They include:
- Autocollimator which is used to measure angular deflections;
- Densitometer for measuring the degree of darkness (the optical density) of a reflecting surface;
- Interferometer for measuring the interference properties of light waves;
- Light meter for measuring the amount of light;
- Photometer for measuring light intensity;
- Polarimeter for measuring dispersion or rotation of polarized light;
- Reflectometer for measuring the reflectivity of a surface or object;
- Refractometer for measuring refractive index of various materials, invented by Ernst Abbe;
- Spectrometer or Monochromator for generating or measuring a portion of the optical spectrum;
- Vertometer which is used to determine refractive power of lenses such as glasses and contact lenses.

Lighter or darker?

The incident white light ray separates into its constituent colours upon refraction

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardized model of human brightness perception. There are different SI units for both systems.
The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use.