EOP - Optical probes with diffuser for irradiance measurements

For measuring irradiance and illuminance as well as for general coupling of light into the spectrometer, Instrument Systems provides a range of optical probes. These differ primarily in the cosine correction, the degree of light throughput and the spectral range. All optical probes include a diffuser for the incident light. Behind the diffuser is the entrance facet of the optical fiber used to couple the light into the spectrometer.

EOP-140, EOP-120, ISP 40


  • flexible optical fiber connector
  • versions with varying cosine correction and light throughput
  • special version with fixed field-of-view of 5.7°

Overview of the available optical probes:

Model Cosine correctionLight throughputSpectral rangeApplication

with fiber bundle connector

EOP-146  goodmedium190 - 2500 nm for extended light sources
EOP-120 mediumgood190 - 1700 nmuniversal
EOP-121 mediumgood190 - 1700 nm universal, flat format housing
EOP-140 lowhigh190 - 2500 nmlow light levels
EOP-542 n/ahigh190 - 2500 nm5.7° field-of-view
with SMA fiber connector
EOP-350  very lowgood1000 - 5000 nmIR spectral range
integrating sphere
ISP40-101excellentlow220 - 2500 nm Spectralon coating for UV measurements
ISP40-102excellentlow240 - 2600 nmBaSO4 coating for a broad spectral range

For general applications we recommend the optical probes from the EOP-120 and EOP-121 series (the latter with fiber bundle connector on side), as these offer the best compromise between cosine correction and light throughput. 

A very good cosine correction is necessary for extended light sources, which unfortunately also means low light throughput - as is the case for the EOP-146.

The optical probe EOP-140 is only recommended for applications that demand a high light throughput. EOP-542 is available for measurements with a specified field-of-view of 5.7°, generally used for measurement of direct sunlight.

Integrating sphere ISP 40

Integrating spheres provide an ideal cosine correction together with a broad spectral range. Instrument Systems offers the ISP 40 to meet this requirement. It was developed for generally collecting radiation and for measurements of irradiance. The ISP 40 is ideally suited for analyzing collimated beams, such as those produced by optical imaging systems, fiber optics and endoscopes. The intrinsic “integrating” function of the sphere means that the entire beam is captured, regardless of whether it is divergent or convergent, and launched into the fiber and subsequently coupled into the spectrometer.


Cosine correction

Assuming that the light source is arranged normal to the detector surface, then according to photometric fundamentals, irradiance can only be correctly determined for an extended source when the signal sensitivity of the detector changes with the cosine of the incidence angle.

E = E0 x cos(α)

E : irradiance at incidence angle (α)
E0 : irradiance at normal incidence
α : angle between the incident light beam and the detector normal

As a general rule, the better the cosine correction, the lower the light throughput.