Irradiance

A relative irradiance measurement corrects the shape of a measured spectrum without defining the scale. It simple scales it from 0 to 1. Calibration is performed by sampling a blackbody light source of known color temperature. The software then uses the color temperature to calculate the theoretical spectral shape of the source and apply a wavelength-by-wavelength correction to the full spectrum.

Calculating relative irradiance looks a lot like a transmission calculation, with the addition of the blackbody reference shape, B(λ).

Relative irradiance measurements work well for measuring the center wavelength of an LED or other limited bandwidth light sources. They can be used in combination with a basic power meter for quality control on production lines to inspect light sources or other emissive sources. A relative irradiance spectrum is also a good starting point for color measurements of an emissive sample like a computer display.

A relative irradiance calibration will work for many applications. It is much more flexible than an absolute irradiance calibration, since any type of sampling optics can be used, and recalibration is quick and easy to do after sampling optics are interchanged.

Any blackbody light source can be used for a relative irradiance calibration. A tungsten halogen light source is a convenient standard, and works well for visible and NIR wavelengths. All of our tungsten halogen light sources have a blackbody emission spectrum, though their color temperatures vary. Be sure to use the correct color temperature, and enter it into the software in degrees Kelvin.

Thanks to easy calibration, any sampling optic can be used for a relative irradiance measurement. The choice of optic will depend on the field of view desired, and the goal of the measurement.

Bare fiber: gives a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. It works well for looking at the light coming from a general area, such as the sky.

Flame loop or other probe: gives the same 25° full angle field of view as a fiber.

Collimating lens: if properly aligned for collimation, it samples a spot size equal to its diameter. Collimation can be checked by routing a light source in reverse through the fiber/lens pair. It works well for sampling light from a specific location at a distance.

Gershun tube kit: allows control over the field of view from 1° to 28° using various apertures and configurations. It works well for sampling light from a specific location at a distance, or with a particular field of view.

Cosine corrector: has a 180° field of view, with a Lambertian response. The amount of light collected will vary as the cosine of the angle of incidence on the cosine corrector. It samples any light incident on the diameter of its white diffusing surface, and specifically in that plane. It works well for looking at how much light is incident on a surface, since ambient lighting or sunlight tends to illuminate surfaces from a wide range of angles.

Integrating sphere: has a 180° field of view at the location and in the plane of its sample port. There is no weighting by angle (unlike a cosine corrector), as all the light that passes through the sample port opening gets sampled by the sphere equally. It works well for looking at how much light per unit area is passing through a specific plane or incident on a surface. It is particularly well suited to measuring the total light output of any source that can be completely enclosed by the sphere, like an LED.

Any spectrometer can be used for relative irradiance measurements, provided it has the right sensitivity for the measurement. We offer a wide range of preconfigured spectrometers, or can create a custom spectrometer for your specific wavelength range and application. Contact one of our application sales engineers to discuss your needs.

Irradiance is the measurement of radiant flux per unit area hitting or passing through a surface. An absolute irradiance measurement results in a spectrum that is accurate in both shape and magnitude. The y-axis scale becomes scaled in power or flux units like μW/nm or μW/cm²/nm, making it easy to calculate other power or energy values.

Measurements in absolute irradiance mode require a calibration using a source with known power output. A spectrum is measured with the sampling optic (fiber tip, CC-3 cosine corrector, etc.) connected to the calibration light source, and is then compared to the known output power of the calibration light source. Remember to always use the light source calibration file specific to the sampling optic being used, and calibrate just prior to measurement if possible.

The calibration process generates a file with energy response data for each pixel in the CCD, given in μJ/count. Factoring in the surface area of the sampling optic and the integration time allows irradiance measurements in μW/cm² to be reported (power=energy/time). Calibration is only possible if the absolute power output of the calibration light source is known, so if the light source calibration data is not given in the units μW/cm²/nm, it may not be a light source capable of calibrating for absolute irradiance measurements.

Calculating absolute irradiance takes into account the collection area of the sampling optic, and is corrected using the calibration data for each pixel, C_P.

 , where

C_p = Calibration file, in μJ/count (specific to the sampling optic)

S _p = Sample spectrum, in counts

D _p = Dark spectrum, in counts

T = Integration time, in seconds

A = Collection area, in cm² (A=1 for an integrating sphere)

dL_P = Wavelength spread (how many nanometers each pixel represents)

An absolute irradiance calibration requires a light source with known spectral output power at certain distance, such as the HL-2000-CAL, DH-2000-CAL, HL-3 or DH-3 series calibration light sources. Each version of these light sources is designed to be used with specific sampling optics (e.g. cosine corrector, integrating sphere or fiber) and a separate calibration file is included for each sampling optic. In addition, the calibration range of these light sources can be extended to 2200 nm by including the EXT option.

Once calibrated, a light source will maintain its calibration for a total of 50 hours of use (including warm-up time). This time limit is due to the natural decrease in the intensity of output of a tungsten bulb over time, and is typical for any calibrated light source. After 50 hours of use, the calibration light source must be recalibrated at Ocean Optics to ensure accurate measurements.

Fewer options are available for sampling optics when making absolute irradiance measurements, but fortunately they address the most common applications and needs. It is critical to check that the calibration source is properly matched to the sampling optic to ensure that the field of view is filled during calibration, and that the correct calibration data is used. For example, power output from a calibration light source will be different when a bare fiber is attached than when a cosine corrector is inserted.

Bare fiber: gives a 25° full angle field of view, sampling a spot size with a diameter equal to ½ the distance to the object or plane of measurement. It works well for looking at the light coming from a general area, such as the sky.

Cosine corrector: has a 180° field of view, with a Lambertian response. The amount of light collected will vary as the cosine of the angle of incidence on the cosine corrector. It samples any light incident on the diameter of its white diffusing surface, and specifically in that plane. It works well for measuring the power incident on a surface, in which case it should be placed flush with that surface for measurement.

Integrating sphere (ISP-50-8-I or ISP-80-8-I): has a 180° field of view at its sample port, and measures all light that passes through its opening. In this configuration, it measures the average radiant flux of a light source at a specific distance, in the plane of the sample port. It is particularly well suited to measuring the total light output of any source that can be completely enclosed by the sphere, like an LED. Similarly, it can measure a light source if its full power can be directed into the port (as in the case of a beam, for example).

Ocean Optics can calibrate on-site with any of the sampling tips described above, as well as with other sampling optics like flame loops, direct-attach cosine correctors, or other integrating spheres. A direct-attach cosine corrector works well for use in the field, where fibers are likely to get damaged, and recalibration is not easy to perform. We can also calibrate a customer-supplied optic provided the collection area is known.

Once in our metrology lab, we will calibrate your system using a NIST-traceable light source for which the irradiance as a function of distance is known. This gives us the flexibility to calibrate a wide range of sampling optics, extended wavelength ranges, and at particularly high and low intensities.

If the calibrated system includes a fiber, care must be taken during use to avoid damage to or stress on the fiber and fiber junctions to maintain the accuracy of the calibration. This is particularly important when an integrating sphere is used for collection, as the integrating sphere is significantly heavier the fiber attached.

Absolute Irradiance Calibration

Absolute Irradiance Calibration

The collection area of the sampling optic is needed to calibrate a system. The software allows this to be entered as a fiber diameter in µm, an area in cm², or as an integrating sphere.

Cosine Corrector with 180° Field of View

Fiber and Cosine Corrector

• If a bare fiber is used, enter the fiber’s core diameter.
• If a CC-3 or CC-3-UV cosine corrector is used, it should be treated as a 3900 µm diameter fiber.
• If a CC-3-DA cosine corrector is used, it should be treated as a 7140 µm diameter fiber.
• If an integrating sphere is used to collect light external to it, enter the port diameter in µm.
• If an integrating sphere is used to enclose the sample and collect all light internally (such as an LED), choose the integrating sphere option; collection area is not applicable in this case.

Any spectrometer can be used for absolute irradiance measurements, provided it has the right sensitivity for the measurement. We offer a wide range of preconfigured spectrometers, or can create a custom spectrometer for your specific wavelength range and application. Contact one of our application sales engineers to discuss your needs.

Once an absolute irradiance spectrum has been measured, it can be used to calculate a lot of other power measurements. Radiometric measurements describe total power (integrated over wavelength). Spectroradiometric measurements describe power as a function of wavelength.

Photopic measurements describe power as perceived by the human eye (weighted by the unique response function of the eye and then integrated over 380 – 780 nm). Scotopic measurements are similar, but describe power as perceived by the human eye under very low light level conditions; the eye then accepts a wider cone of light, hitting different receptors and altering the response curve of the eye.

Absolute irradiance data can also be used to calculate the number of photons, moles of photons, dBm (for lasers), and eV. Even photosynthetically active radiation (PAR) can be calculated. Many of the values described above can also be integrated over a user-defined wavelength range using the software. Regardless of the value being calculated, it is always a good idea to save the full irradiance spectrum so that the data can be reanalyzed later, in a different way if needed.

* sr = steradian, a unit of solid angle; 4π steradians is a sphere

• If possible, calibrate the system on location, just prior to measurement.
• Allow the calibration light source warm up for at least 20 minutes. Always calibrate with any optics and fibers that will be in the path during the measurement.
• Use a high number of averages, particularly when performing the calibration scan, to achieve the best results.
• Always keep a log of calibration light source use, including date of last calibration and subsequent hours of operation (including warm-up). Light sources must be recalibrated after 50 hours of use to maintain accuracy.
• Validate the absolute irradiance calibration by measuring the calibration light source once and then overlaying it with the light source calibration file. You can then hide the light source calibration using the same menu item.
• Don’t change the boxcar smoothing value after it is set during calibration.
• Though integration time and averaging may be changed while making measurements, do remember to store a fresh dark spectrum with those parameters. The software will take care of the rest.

Upwelling is radiation from the earth’s surface, whether reflected solar light or emitted terrestrial radiation. Downwelling radiation is directed toward the Earth’s surface from the sun or atmosphere. The relationship between the two (albedo) can be used to derive spectral information from vegetation, forest canopies, sea beds and more that aid in ecological and photosynthesis studies.

An extended range spectrometer like the HR4000 with an HC-1 grating and 50µm slit covers a 200 – 1050 nm wavelength range with ~1.5 nm resolution (FWHM), giving enough sensitivity, resolution, and range to properly study this type of radiation. A two-channel Jaz system can be used as an alternative, both for portability and to allow upwelling and downwelling measurements to be captured simultaneously.

The two most popular sampling optics chosen for this application are a cosine corrector and a Gershun tube.  A CC-3-UV cosine corrector gives a full 180° field of view, while a Gershun tube allows a specific field of view to be chosen (in variable increments from 1° to 28°). Either can be used in combination with a calibration source and software to calculate spectral intensity and photopic data in lumens, lux or candela.

LEDs are used extensively in industry for uses as varied as indicator lights and biomedical devices. Characterizing the output power, wavelength, and spectral shape of LEDs is an important part of their manufacture and binning for end use. An integrating sphere that encloses the LED is the simplest way to capture all emitted light and exclude ambient light from testing.

An FOIS-1 integrating sphere and spectrometer mated with an LED-PS power supply provides both the electronics to consistently drive single, undeployed LEDs under test and measure their output accurately. This integrating sphere can be calibrated using an HL-3 series calibration lamp for integrating spheres, creating a system capable of measuring photopic output in lumens, dominant and peak wavelength, and CIE color coordinates of LEDs.

If the LEDs have already been integrated into a subassembly or system, a spectrometer with a cosine corrector may be more appropriate to measure color and output at a specific distance from the LED. This type of system can even be made fully portable using a Jaz spectrometer with battery module.

Solar irradiance measurements are critical to a variety of fields of research, including atmospheric studies of greenhouse gases and ozone depletion, the effect of solar radiation on ecological systems and crops, and effects of UV sunlight on skin and eyes. Measurements of upwelling and downwelling radiation are also important in ecological and photosynthesis studies.

Solar radiation spans a wide range of wavelengths, from UV to NIR. Maximum coverage of this range can be achieved using a preconfigured HR spectrometer (200 – 1100 nm) or a JAZ-EL200 (200 – 1025 nm). Of the two, the Jaz offers much greater portability, with extended battery life and on-board processing. A Jaz spectrometer calibrated on-site at Ocean Optics with a direct-attach cosine corrector even carries its calibration data with it on an SD card that fits into the battery module. It is also Ethernet-capable, allowing data to be reported back to a field station, or a laboratory in another country.