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Enabling better global research outcomes in soil, plant & environmental monitoring.

ULM 500 Universal Light Meter & Data Logger

Originally, Walz produced PAR-sensors as accessories for PAM Chlorophyll Fluorometers. Now, Walz also offer a fast data logger with a high dynamic range and adequate PAR-sensors for independent PAR-measurements or PAR-monitoring.

The ULM-500 data-logger and sensors are capable of resolving the saturating light pulse of PAM Chlorophyll Fluorometers or fast light flecks in nature.


Light Measurement

The main purpose of the ULM-500 is light-measurement. The ULM-500 has two connectors for light sensors with BNC plugs. Typically, PAR sensors and pyranometers are linked to the ULM-500. PAR sensors measure the flux of quanta (unit, µmol m-2 s-1) in the photosynthetically relevant wavelength range, that is, between 400 and 700 nm. Pyranometers detect the power of radiation (unit, W m-2) mostly in a wider spectral range so that solar radiation arriving at the Earth’s surface can be measured. When power of radiation is measured, a flux of short-wavelength (e.g. blue) photon results in a higher signal than the same flux of photons with longer wavelengths (e.g. red) simply because the former flux carries more energy than the latter.

The higher energy in blue than in red photons, however, is unavailable for photosynthesis so that photon fluxes as they are measured by PAR sensors are important for photosynthetic studies. We offer three different PAR-sensors suitable for use in conjunction with the ULM-500.

Dynamic Range

The ULM-500 has 5 sensitivity ranges with fast automatic switching. As a result it has a high dynamic range reaching up to 99 999 µmol m-2 s-1 and may detect signals down to 0.1 µmol m-2 s-1 – even with the smallest sensor we offer. The range-switching can be set to automatic for convenient measurements or to manual for fast time resolution.

High time resolution for the record of saturation pulses of PAM instruments (here the SP of a Phyto-PAM is shown)

Time Resolution – Chart Mode

Channel no. 1 of the ULM-500 is optimised for high time resolution. In the chart mode a time course of 1.2 s (or 2.4 s) is shown, which contains 120 data points (max. 100 Hz). The minimum, maximum and average are indicated. With the triggered chart function a saturating light pulse changing from dark to several thousand µmol m-2 s-1 can be resolved. This is usually not possible with standard light meters. The ULM-500 can be connected to the software WinControl-3 for continuous fast recording of light flecks.

Data Storage

The ULM-500 can be used as simple light meter for single light measurements or as data logger for continuous measurements. It can store up to 50 000 data points. Stored data can be downloaded with the software WinControl-3.

Long term recording of slow light changes with ULM-500 (and a cosine corrected sensor as PAR detector)


The ULM-500 has two BNC-connectors for light or PAR-sensors. Furthermore it has one connector for the Monitoring Leaf-Clip JUNIOR-B, which can also be used to connect the Leaf-Clip 2030-B via an additional adapter. With the leaf clips not only PAR but also leaf temperature is measured and displayed.


The ULM 500 can be operated with its internal batteries or power can be supplied via USB-connector. Display illumination with white LED backlight renders readability in the dark. The ULM-500 is a stand-alone instrument, but can also be connected to a computer and operated with the software WinControl-3. Storage of several sensor calibration data is possible (10 different calibration values with probe names via WinControl-3 software).

Design: Light grey plastic housing with connectors, membrane keyboard and a white illuminated LCD graphic display
Dimensions: 12 x 7.5 x 3.5 cm
Weight: 210 g (including 4 AAA 1.5 V batteries)
Power supply: 4 AAA-type batteries or 5 V DC from USB voltage source when connected to the computer
Working conditions: 35 to 85 % rH (avoid condensation), – 20° to + 50°C ambient temperature
Time resolution: PAR channel #1: 100 samples / second, PAR channel #2 and other channels: 5 samples/second
Operation time: 10 days or ca. 100 days automated logging with sleep mode (1 meas. / 5 min). Unlimited working time via USB connection (PC-software WinControl-3 – no sleep mode)
  • Two BNC-connectors for the connection of two PAR-sensors with individual calibration factors between -50.0 and -9999.9 (memory for 10 calibration factors), range switchable in 5 steps 250 nA to 0.6 mA
  • Connector for Monitoring Leaf-Clip JUNIOR-B; an adapter is available for the connection of the Leaf-Clip Holder 2030-B, and the Micro Quantum/Temp.-Sensor 2060-M
  • Connector for additional digital sensors (still under development)
  • USB-Connector for connection with computer (software: WinControl-3)
Memory: Flash memory used as ring buffer for 50000 lines (1 line / single measurement)
Display: White illuminated graphic display with 5 different display modes (1: all data; 2-4: two selected sensors in big letters; 5: chart mode for channel no. 1, with maximum, minimum and average indicated), resolution: 0.1 μmol m-2 s-1
Computer-controlled Operation: 1 free USB socket. Processor, 1 GHz. RAM, 256 MB. Hard disc space, 20 MB. Screen resolution: 800 x 600 pixels. Interface, USB 1.1, 2.0 and 3.0. Operating system: Microsoft Windows XP/Vista, Windows 8.
  • LS-C Mini Quantum Sensor
    The Mini Quantum Sensor LS-C is a miniaturised sensor designed to be placed close to leaves in small spaces, for example within cuvettes. The LS-C measures photosynthetically active radiation (PAR).
  • MQS-B Cosine Corrected Mini Quantum Sensor
    The Cosine Corrected Mini Quantum Sensor MQS-B is designed for light measurements in units relevant for plant leaves.
  • US-SQS/L Submersible Spherical Micro Quantum Sensor
    When taking measurements under water or when working with suspensions, the amount of scattered light is very high. To compensate for this, a spherical sensor should be used. The US-SQS sensor uses a plastic diffuser to obtain an angular response error of less than ± 5% from -100° to 100° angle.