Light measurement units are defined around very specific spatial responses. A carefully designed input optics system is essential if measurement accuracy is to be achieved. Two laws of geometrical optics apply directly to radiometry and photometry: the inverse square law and the cosine law.

The inverse square law defines the relationship between irradiance from a point source and the distance to the measurement surface. It states that the intensity per unit area varies in inverse proportion to the square of the distance between the source and surface. In other words, if you measure 16 W/cm² at 1 meter, you will measure 4 W/cm² at 2 meters, and can calculate the irradiance at any other distance. A well defined measurement plant and approximate point source are required for valid interpolations.

In radiance measurements (W/cm²/sr), where a detector with a narrow viewing angle looks at a much larger, uniform area source, the area of the source that the detector sees increases as the detector moves farther away, counteracting the effects of the inverse square law and making the reading independent of distance. Radiance measurements are useful for characterizing large area sources such as CRT's and backlit displays, or for diffuse, uniformly illuminated surfaces.

The cosine law refers to the relationship between the irradiance on a surface and the incident angle. The intensity falls off as the cosine of the angle, since the effective surface area is reduced as the angle increases. Irradiance and Illuminance detectors, especially those with filters that restrict off-angle light, require cosine correction to properly quantify light coming form all angles. Inaccurate cosine response is one of the biggest sources of error in full immersion applications.

There are many applications in light measurement that require specialized input optics. For example, intense laser beams must be diffused, so that the narrow beam does not locally saturate the receiver. Many high intensity ultraviolet sources require temperature resistant remote optics that offer cosine spatial responses and side viewing.

We offer a wide variety of standard input optic choices, and we can also design a custom solution for your application. Contact us for further information.

W "Wide Eye" Diffuser: an internal hemisphere ground in solid quartz for rugged durability and wide spectral transmission. The TD Diffuser, with its external teflon hemisphere, is a lower cost solution for use in UV and visible applications. Both provide a near perfect cosine response, necessary for accurate irradiance measurements.

R Radiance Barrel: Employs dual imaging lenses and internal baffling to provide a ±0.75 degree viewing angle with a 25 mm lens aperture. Radiance measurements are independent of distance when the viewing angle does not exceed the area of the source. Radiance is a measure of flux density per beam solid angle.

L30 High Gain Lens: Increases the sensitivity of the SED033/W by a factor of 40, while limiting the field of view to ±8 degrees. This is often desirable when measuring low level point sources. The L30 is particularly useful in flash measurements, where the amount of energy in the pulse is very low, and radiates from a well defined point source.
LENS DIAMETER: 37 mm
RING DIAMETER: 2" (50.8 mm)

HNK15 narrow Beam Adapter: Combination of an H hood barrel, N narrow beam adapter with three quartz diffusers, an internal integrating chamber, and a K15 cone aperture, produces an effective 10 mm diameter input aperture with uniform translation response to eliminate beam aiming errors and provide beam diffusion and attenuation.

H: Hood Barrel: this 62.5 mm long by 42 mm diameter internally baffled barrel is useful for eliminating stray light, reducing the field of view to 23°. Accepts standard filters.

SR, S1W, S2W: Spectroradiometer Optics: SR: 8° field of view. S1W: 15° field of view. S2W: 35° field view. Quartz provides a wide spectral range.

FFO2500: Quartz Fiber: This 2 mm diameter x 500 mm long fiber optic guide has 28° acceptance angle and passes UV (220 - 1400 nm). 120°C max operating temperature.

K9, K15: Cone Apertures: These 9 and 15 mm diameter apertures restrict incoming light to the detector surface area for beam aiming applications. Used primarily as a laser beam target aperture.

RAA: Right Angle Adapter: Input adapter for fiber optic cables includes internal quartz mirror and W Diffuser for cosine spatial response in harsh measurement environments. 36 mm diameter x 25 mm.

P2, P6, P8, P9: Fiber Optic Adapters for fiber probe.

P6 = 12.5 mm aperture
P2 = 4.3 mm aperture
P8 = 8.3 mm aperture
P9 = 9.2 mm aperture. The P8 is a larger holder designed for the RAMP.

A312, A313: Flat Diffusers: A312 - Flashed Opal, A313 - Flat Quartz, for diffusing laser beams, fiber optic inputs in applications requiring internal diffuser. External threads for input optics.

T, T2X: Flat Teflon Diffusers: T - Flat Teflon for secondary diffusion. T2X - Teflon mounted with filter in primary diffuser ring. Excellent cosine diffusion for high throughput, maximum sensitivity.

LED: LED Intensity Barrel: The LED Barrel measures LED luminous intensity in millicandela. The P2 accepts 2 mm fiber optic tips in LEDs. Custom inserts are available, up to 6 mm diameter.

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