Kriptometre

What is a kriptometre?

A kriptometre, more commonly spelled cryptometer and sometimes kryptometer, is a traditional instrument for measuring the intensity of solar radiation. Despite the ending “metre,” it is neither a length nor a modern SI unit. It is a device that indicates how much radiant energy reaches a detector under defined conditions.

The term is now uncommon in contemporary technical literature. It may appear in historical scientific catalogues, older laboratory manuals, instrument collections and educational material. Modern solar measurements are more often described as radiometry or photometry, depending on the detector and the quantity being measured.

How does a kriptometre work?

A cryptometer allows a controlled quantity of sunlight to reach a sensitive detector. Depending on its design, the detector may convert the radiation into an electrical, thermal or photographic signal. That response is then compared with a calibrated reference or converted through a calibration curve into a radiation value.

The instrument’s optics and filters determine exactly what it measures. Some arrangements emphasize direct sunlight, while others include much of the sky. Consequently, two cryptometers may produce different readings even when used side by side if their spectral responses, viewing geometries or calibration methods differ.

What does it measure?

The historical word “intensity” can be misleading because it does not identify a single modern unit. Related instruments may report:

• Radiant irradiance, expressed in watts per square metre, when the response covers the relevant solar spectrum.
• Visible-light illuminance, expressed in lux, when the detector is weighted to approximate human visual sensitivity.
• Relative brightness, when the instrument is intended only for comparing readings taken with the same device.

These quantities are not interchangeable. A reading in watts per square metre cannot be converted into lux without information about the spectral distribution of the light. A useful measurement therefore depends on clear documentation of the detector, calibration and test conditions.

Where is it used?

Historically, cryptometers were associated with meteorological observation, solar research and experimental physics. They could also be used for demonstrations involving sunlight, radiant energy and the response of different materials. Today, the name is more likely to be encountered in museum collections, historical catalogues or discussions of early photometry than in new solar-energy installations.

Modern instruments have largely replaced them because they offer better calibration traceability, spectral control, digital output and automatic data logging. Nevertheless, the cryptometer remains useful for understanding how solar radiation was measured before these integrated systems became standard.

Choosing an instrument for solar measurements

For a contemporary application, consider the following before buying or using a solar radiation instrument:

• The required wavelength range and whether the measurement should cover direct, diffuse or total radiation.
• The field of view and angular response, especially for distinguishing direct sunlight from the whole sky.
• Calibration traceability to a recognised reference.
• Temperature compensation, because detector response can change as the instrument warms.
• Compatibility with data loggers and the required sampling interval.
• Availability of replacement parts, documentation and periodic recalibration.

Practical care and safety

Keep the optical window clean and free from dust, fingerprints and