With today's electronic equipment level, each of these
requirements are achievable quite easily, but all together it is
difficult task. A key problem is the measurement of small
levels of illuminance.
Base for luxmeter with high sensitivity (the ability to
evaluate illuminances about 10-2 lx) is a high quality sensor
adapted to the sensitivity of the human eye curve. If the sensor
is not good enough, you can not evaluate objectively
illuminances by the best electronic circuits.
For linear transfer of illuminances to the electrical units
from very small illuminances values it is evaluated current and
it is preferred in short state. This can be realized by currentvoltage
converter with an operational amplifier. Currents,
which the converter operates with at the illuminance up to 1lx
are very small (approximately – nA). Operational amplifier for
these currents must be able to evaluate very small input,
preferably by several degree orders smaller than the operating
current. These requirements meet the operational amplifiers
with FET input. At the same time it needs to have a minimum
noise and drift and it must be able to work with low supply
voltage (the requirement for a battery). It was chosen
operational amplifier AD822. To reduce the noise (to provide
sufficient accuracy) an analogue part of the photometer is
shielded.
B. Description of luminance analyzer LMK mobile advanced