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UV & Visible Light
The 764 calibration should not drift with time but to be sure of accuracy the
instrument can be returned to the manufacturer for a calibration check every 2-5 years.
The 764 has a very carefully defined frequency response (i.e. which wavelengths of
light it is sensitive to). Other light meters are often more sensitive to infra red light
than they should be and so will give a higher reading with light sources that contain
infrared (e.g. ordinary filament light bulbs).
Because the 764 has a cosine angular response it may give different readings to a type
762 which is relatively directional. In other words the 764 is more sensitive to light
coming at an oblique angle than the 762.
Note that the human eye has a logarithmic response, this means that if two luxmeters
are placed side by side they may give different readings even if it looks like they are
getting a similar amount of light. To compare the readings on 2 meters they each need
to be placed in the exactly same position under exactly the same lighting conditions
with the operator being very careful not to shade the meters differently.
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