The LM631 digital light meter is a portable, easy-to-use, 3.5-digit, compact-sized digital light meter designed for simple one-handed operation reading in lux or fc units. Pressing the PEAK button for more than 2 seconds turns the backlight on and off. There is also an auxiliary output at the microphone jack (2.5 mm) connector that is active as long as the light meter is turned on.
The LM631 is a compact, portable digital light meter with a 3½‑digit LCD display and backlight. It measures illumination in lux or foot‑candles, offers a peak‑hold function, and can be operated with one hand. Pressing the PEAK button for more than two seconds toggles the backlight.
The manual lists the instrument’s performance ranges (20–20 000 lux or fc), accuracy, temperature coefficient, resolution, and spectral response. It also details power requirements (four AAA batteries lasting about 200 hours), auto‑power‑off after 15 seconds, standby current below 1 µA, dimensions (170 × 44 × 40 mm), weight (220 g), and included accessories such as a carrying case and lens cover.
| Light Intensity Range | 20–20 000 lux; 20–20 000 fc |
|---|---|
| Total Accuracy | + (3 % reading + 10 digits) |
| Temperature Coefficient | 0.1 × specified accuracy/°C (< 18 °C or > 28 °C) |
| Resolution | 0.01 lux; 0.01 fc |
| Spectral Response | CIE photopic – CIE standard illuminant A |
| Acceptance Angle | < 2 %; cosine corrected (150 °) |
| Peak Hold Response Time | > 50 ms pulse light |
| Display | 31‑digit LCD, max 1999, over‑range indicator (OL) |
| Measurement Rate | 2.5 times per second (nominal) |
| Operating Environment | 0 °C to 50 °C (32 °F to 122 °F) at < 75 % RH |
| Storage Temperature | -20 °C to 60 °C (-4 °F to 140 °F), 0‑80 % RH (batteries removed) |
| Altitude Rating | Up to 2000 m |
| Battery | 4 × 1.5 V AAA (UM‑4, R03); ~200 h life |
| Auto Power‑Off | ≈ 15 seconds |
| Standby Current | < 1 µA |
| Dimensions | 170 × 44 × 40 mm |
| Weight | 220 g |
Press the PEAK button for more than two seconds to toggle the backlight.
It measures illumination from 20 lux (or 20 fc) up to 20 000 lux (or 20 000 fc).
The four AAA batteries provide about 200 hours of operation.
The total accuracy is + (3 % reading + 10 digits) for CIE standard illuminant A.