EXTECH 403125

User's Guide
Digital Light Meter
Model 403125
EXTECH INSTRUMENTS CORPORATION warrants this instrument to be free of defects
in parts and workmanship for one year from date of shipment (a six month limited warranty
applies to sensors and cables). If it should become necessary to return the instrument for
service during or beyond the warranty period, contact the Customer Service Department at
(781) 890-7440 ext. 210 for authorization or visit our website www.extech.com for contact
information. A Return Authorization (RA) number must be issued before any product is
returned to Extech. The sender is responsible for shipping charges, freight, insurance and
proper packaging to prevent damage in transit. This warranty does not apply to defects
resulting from action of the user such as misuse, improper wiring, operation outside of
specification, improper maintenance or repair, or unauthorized modification. Extech
specifically disclaims any implied warranties or merchantability or fitness for a specific
purpose and will not be liable for any direct, indirect, incidental or consequential damages.
Extech's total liability is limited to repair or replacement of the product. The warranty set
forth above is inclusive and no other warranty, whether written or oral, is expressed or
Congratulations on your purchase of the Extech 403125 Digital Light Meter. This
portable, compact light meter provides lux and fc measurements. This meter includes a
backlit LCD display and offers PEAK HOLD and DATA HOLD functionality. This meter is
shipped fully tested and calibrated and, with proper use, will provide years of reliable
Safety Instructions
It is recommended that the user read and understand the safety and operational
instructions before using the light meter. Please read the following WARNINGS:
To avoid electrical shock do not operate this meter in wet or damp conditions.
Do not immerse the meter in liquid. For cleaning instructions, refer to the
maintenance section of this manual.
To avoid injury and fire hazard, do not use this meter in potentially explosive
To avoid eye injury, wear eye protection if there is the possibility of exposure
to high intensity light rays.
Cover the sensor head when not in use. This will extend the life of the photodiode employed in the sensor head.
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General Specifications
3 ½ digit backlit LCD display (reads 0 to 1999)
Over range indication
OL (over load) is displayed
Measurement rate
2.5 readings per second (nominal)
Battery power
Four (4) 1.5V ‘AAA’ batteries
Battery life
200 hours typical
Low battery indication
Battery symbol is displayed when the battery power drops
below the operating voltage
Operating conditions
32 to 122 F (0 to 50 C) < 70% Relative Humidity
Storage conditions
-4 to 140 F (-20 to 60 C) < 80% Relative Humidity (with
batteries removed)
Analog output
0.1mVDC per display digit
6.7 x 1.7 x 1.6" (170 x 44 x 40mm)
Approx. 7.76 oz. (220g) with batteries
Electrical Specifications
20, 200, 2000, 20,000 fc and lux (note that the 20,000 range is
displayed in the x10 mode, meaning that the display must be
multiplied by 10).
0.01 lux/fc maximum
20, 200 ranges: ± (5% of reading + 10 digits)
2000, 20,000 ranges: ± (3% of reading + 10 digits)
Accuracy note: Stated accuracy for ± 9 F at 73 F (±5 C at
23 C) < 70% Relative Humidity
Spectral response
CIE photopic
Note: The CIE photopic curve is an international standard for
the color response of the average human eye. The CIE
standard illuminant ‘A’ is defined as a gas-filled Tungstenfilament lamp operating at a correlated color temperature of
Peak Hold response
50ms minimum response time for the measurement of a light
Acceptance angle
f ’2 < 2% cosine corrected (150 )
Units conversion
10.764 fc = 1 lux (lumens/meter )
0.0929 lux = 1 fc (lumens/foot )
Temperature Coefficient
0.056 times the specified accuracy per F (< 64.4 F or >
82.4 F)
0.1 times the specified accuracy per C (< 18 C or > 28 C)
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Meter Description
LCD display
and PEAK button – Momentarily press this button to
activate/deactivate the Peak Hold function. The PMAX display icon will appear.
In the Peak Hold mode of operation, only the highest reading will be displayed.
Press and hold this button for 2 seconds to switch on the backlight. Press and
hold this button again for 2 seconds to turn the backlight off.
RANGE button – Press to select the
desired lux or fc range. Each press of the
RANGE button selects a new range. The
available ranges are 20, 200, 2000, and
20,000 lux or foot candles. Displays in
the 20,000 range must be multiplied by
HOLD button – Press to freeze the
displayed value (the H icon will switch
on). Press again to exit the Data Hold
Sensor cable – Connects sensor to meter
Sensor (sensor cover not shown)
Sensor mounting plugs - Fits into docking
jacks at top of meter
Docking jacks – Sensor can be attached here
for storage or while measuring.
Function switch – Three positions OFF, LUX,
and FC
Handle and battery compartment (rear)
Analog output jack
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1. Set the function switch to the desired unit of measure; lux or fc.
2. Remove the light sensor cover.
3. Hold the sensor head steady or place it in a secure area and allow the sensor to collect
light. The light must encompass the entire sensor dome. Ensure that the user does not
cast a shadow on the sensor dome. The sensor is connected to the meter by a 4’
(1.5m) cable. While measuring, the sensor can be kept docked on the meter (as it is
shipped) or it may be disconnected and used remotely.
4. Read the illuminance value on the display. If the approximate magnitude of the light is
not known, step through the ranges using the RANGE button until a measurement is
displayed. If x10 appears on the LCD, the meter is in the 20,000 range and the reading
must be multiplied by 10. The OL (overload) icon appears if readings are out of range.
5. While taking a measurement, press the HOLD button to freeze a displayed reading.
Press HOLD again to release the displayed reading. The H icon will appear on the
display when the meter is in the HOLD mode.
6. If it is desired to know only the highest reading of a given measurement period, press
the PEAK button at the start of a measurement period. The PMAX display icon will
switch on in the PEAK mode and the display will only show the highest reading.
Momentarily press the PEAK button again to exit the PEAK mode.
7. Press and hold the PEAK button for seconds to activate the display backlight. Press
and hold the PEAK hold button again for 2 seconds to switch the backlighting off.
8. Cover the sensor when not in use.
Measurement Considerations
1. For best results, repeat measurements several times to ensure that the light source
has remained stable.
2. Avoid flexing the cable excessively at either end.
3. The Inverse Square Law: The law states that the illuminance E at a point on a surface
varies directly with the intensity I of a point source, and inversely for the square of the
distance d between the source and the point. If the surface at the point is perpendicular
to the direction of the incident light, the law is expressed as E = I / d .
4. Cosine Law: The law states that the illuminance on any surface varies with the cosine
of the angle of incidence. The angle of incidence θ is the angle of the surface plane
and the direction of the incident light. The inverse-square law and the cosine law can
be combined as follows: E = (I cos θ) / d .
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CIE Photopic Curve
Analog Output
The meter can be connected to a chart recorder or other analog recording device via the
output jack at the bottom of the meter. A mono (two wire) 2.5mm plug and cable (not
supplied) are required. The analog output is 0.1mVDC per display digit.
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Battery Replacement
When the battery power falls below the operating voltage, the battery symbol appears on
the display. If the battery symbol appears on the LCD or if the LCD does not switch on
when the meter is turned on, replace the batteries as follows:
1. Remove the Phillips screw on the back of the meter.
2. Open the battery compartment.
3. Replace the four (4) ‘AAA’ 1.5V batteries observing polarity.
4. Replace the battery compartment cover and secure the compartment screw.
Use only a lightly damp cloth to clean the meter. Do not use abrasives, solvents, or harsh
detergents. Keep the plastic sensor dome clean and free from scratches; it may be
cleaned with a soft cloth and isopropyl alcohol.
Typical Light Levels (for general reference only)
Foot candles
> 10000
> 107600
Typical Light Levels
1000 to 10000
10760 to 107600
Operating Room
Assembly (fine)
100 to 1000
1076 to 10760
Football stadium
Emergency room
Drafting table
Overcast day
Assembly (general)
10 to 100
Casual reading
107 to 1076
Shipping area
ATM machine
1 to 10
Storage room
10.7 to 107
Building entrance
Parking lot
0 to 1
0 to 10.7
Full moon
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Calibration and Repair Services
Extech offers repair and calibration services for the products we sell. Extech also
provides NIST certification for most products. Call the Customer Service Department for
information on calibration services available for this product. Extech recommends that
annual calibrations be performed to verify meter performance and accuracy.
Support line (781) 890-7440
Technical support: Extension 200; E-mail: [email protected]
Repair & Returns: Extension 210; E-mail: [email protected]
Product specifications subject to change without notice
For the latest version of this User’s Guide, Software updates, and other
up-to-the-minute product information, visit our website: www.extech.com
Extech Instruments Corporation, 285 Bear Hill Rd., Waltham, MA 02451
Copyright © 2006 Extech Instruments Corporation
All rights reserved including the right of reproduction in whole or in part in any form.
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