Chapter 5 ___________________________________________Theory of Operation
VAISALA__________________________________________________________27
CHAPTER 5
THEORY OF OPERATION
This chapter describes the theory of operation of LM21
Background Luminance Sensor.
Functional Description
LM21 Background Luminance Sensor measures the amount
of light coming from an angle of 6° to a 24-mm diameter
lens. The incoming light is focused on a photodiode.
The light spectrum detected by the PIN photodiode is
narrowed by a green filter to resemble the spectral response
of the human eye.
The photoelectric current is measured and converted into
cd/m² or to a frequency that linearly corresponds to cd/m²
divided by 10.
The LM21 sensor consists of a cast aluminum housing with
an integrated hood and mounting accessory, an optics
module with a window, filter, and a lens, and a connection
cable to the host computer. An electronics board is fastened
to the optics module. Refer to Figure 6 on page 28.
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