IRpass

IRpass is a term used in optics and imaging to denote an infrared-pass characteristic, meaning that electromagnetic wavelengths in the infrared region are transmitted while shorter wavelengths are attenuated. In practice, IRpass often refers to infrared long-pass filters, which have a cutoff wavelength and begin to transmit wavelengths longer than that cutoff. The spectral transmission curve gradually rises from near the cutoff, and the filter may be paired with coatings to suppress out-of-band light and reduce stray thermal radiation.

Typical IRpass components include glass or crystal substrates with dielectric or metal-dielectric coatings. Common cutoff wavelengths for infrared pass filters range from about 700 nm to 1500 nm and higher, depending on the application. In camera optics and astronomical instrumentation, IRpass filters improve contrast by reducing visible light and allowing infrared signals to reach the sensor. In spectroscopy, IR pass elements help isolate infrared emission lines from a sample.

When selecting an IRpass filter, specifications such as cutoff wavelength, cutoff sharpness (defined as the slope of the transmission transition), maximum transmittance in the passband, blocking of shorter wavelengths, and environmental durability are considered. The term may also appear in detector and imaging-system descriptions to indicate a system’s infrared-transmission window rather than a discrete physical filter.

See also: optical filters, long-pass filters, bandpass filters, infrared spectroscopy, thermal imaging.