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To disable this function, press and hold the
pressing the
button. Release both buttons and the LM192 will
perform the self-test. The
symbol will disappear from the LCD to
confirm the auto power off is disabled.
3.6 Backlight
Press the
button to turn on the display backlight. It will
automatically turn off after 30 seconds if there is no further
operation of the unit.
3.7 Min/Max
MIN
Press the
button to enable the MIN/MAX mode. By pressing
MAX
MIN
the
button, the user can select between the minimum,
MAX
maximum and current readings. If the LM192 is to be left monitoring
lux levels over a period of time greater than 30 minutes, please
note that Auto power off function will need to be disabled first (see
section 3.5). Failure to disable the Auto power off will result in the
LM192 switching off while recording lux levels. To exit MIN/MAX
MIN
mode, hold the
button for 3 seconds and the MIN & MAX
MAX
symbols will disappear to confirm the function is disabled.
3.8 Hold Mode
To enter the HOLD mode, press the
measurement mode. The HOLD symbol will appear on the LCD. To
HOLD
exit display hold, press the
button again.
3.9 Unit of Measurement
fc /lx
Press the
button to change the unit of measurement between
Foot Candle (fc) and Lux (lx).
The Inverse-square Law
The illuminance E at a point on a surface varies directly with the
intensity I of a point source, and inversely as the square of the
distance d between the source and the point. If the surface at
the point is normal to the direction of the incident light, the law is
2
expressed by E = I/d
.
Cosine Law
The illuminance on any surface varies as the cosine of the angle
of incidence. The angle of incidence
normal to the surface and the direction of the incident light. The
inverse-square law and the cosine law can be combined as
ѳ
E = (I cos
)/d
2
.
Spectral Mismatch Correction Factor
Spectral mismatch is the result of the differences between the
relative spectral responsivity of a light meter's sensor and the CIE
HOLD
button while
HOLD
button while in
7
ѳ
is the angle between the
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3.10 Auto/Manual Ranging
The LM192 defaults to Auto range and selects the lowest range
available. The user can select a different measuring range by
pressing the
RANGE
button.
The available ranges are:
AUTO, 200, 2000, 20000, 200000 lx
AUTO, 20, 200, 2000, 20000 fc
3.11 Measurement Mode
Hold the LM192 so the light sensor is directed toward the light
source.
Press
fc/lx
to select lux or fc as required.
Read the measured light intensity from the display.
3.12 Measurement Considerations
Keep the plastic domed sensor clean and free of scratches. It may
be cleaned with a soft cloth and isopropyl alcohol.
The light level measurement should be taken at the application
height. For light level measurements at a work station, the LM192
should be positioned at desk height. When measuring a work area or
corridor, it should be placed on the floor.
It is important that the light source completely fills the sensor with
minimal movement of the meter during measurement.
It is also advisable where possible to hold the LM192 away from
your body when taking measurements to avoid reflections or creating
a shadow on the sensor. The light sensing probe can be detached
from the LM192. Avoid flexing the cable excessively at either end of
the cable. For best accuracy, repeat the measurement several times
to ensure that the light source has remained stable.
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photopic curve it is designed to simulate, when the light meter is
used to measure a light source with a different relative spectral
power distribution to that of the light source used to calibrate the
light meter.
The LM192 has been calibrated against a standard tungsten light
source at a correlated colour temperature of 2856K.
Where the best possible accuracy is required when measuring
another light source type, it is suggested the LM192 should be
calibrated for that light source type so as to determine the spectral
mismatch correction factor (previously known as colour correction
factor) for that light source type.
3.13 CIE Photopic Curve
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