The range of reflectance is achieved by mixing black pigment into the Spectralon at varying concentrations.

Spectraflect is applied by spraying the coating onto a specially prepared surface. The typical thicknesses value is approximately 0.5 mm (0.020 inches).

Spectraflect® coating can be applied to virtually any substrate and is an ideal reflectance coating for items such as optical components, integrating spheres, lamp housings and spectral diffuser panels.

Generally, Aluminum, Nickel, Steel and Copper.

Please refer to the “Spectralon® Reflectance Standards Care and Handling Guidelines,” which can be found in our Technical Documents Library under Materials and Coatings.

Unless otherwise specified, all data given for Spectralon would be for a thickness greater than, or equal to, 7 mm. As illustrated in our "Guide to Reflectance Materials and Coatings," at a thickness of 7 mm, Spectralon has become nearly opaque with only a small percentage of incident light being transmitted. If thickness is less than 7 mm, Spectralon starts to become translucent.

Signal limits depend upon the:
- Type of detection system
- Source wavelength / range

Typical limits* for broadband detectors are:
- For a silicon detector in VIS: 10 mW (ROM estimate)
- For a germanium detector in NIR: 100 mW (ROM estimate)

Other factors include:
- Fiber optic coupling increases the minimum by about 1000x
- Spectralon vs. Spectraflect increases throughput
- 30% in VIS
- 200% - 300% or more in NIR

It is best to consult Labsphere if applications approach these limits.

Thermal limits depend upon the:
• Amount of Flux absorbed & converted to heat
• Ability of Sphere to dissipate heat

Typical limits are:
- for a 2-inch integrating sphere, 25W
- for a 6-inch integrating sphere, 225W

It is best to consult Labsphere if applications approach these limits.

MAXIMUM power is determined by THERMAL limits, MINIMUM power is determined by SIGNAL limits. Since initial flux is concentrated in a small area, the MAX maybe determined by the LASER DAMAGE THRESHOLD.

For single wavelengths, 350 nm – 1800 nm depending on the detector.

For multiple wavelengths:
• With a silicon detector, 350 nm – 1100 nm, every 25 nm.
• With a germanium detector, 800 nm – 1800nm, every 25 nm.
• With an InGAS detector, 900 nm - 1700 nm, every 25 nm.

The Labsphere integrating sphere for laser power measurement has taken many of the issues found with lasers and designed a sphere to overcome those problems so the sphere is:

• Insensitive to beam divergences up to 40° half-angle.
• Insensitive to input beam alignment up to 40° off normal.
• Insensitive to polarization
• Provides the accurate measurement of beams up to 0.9 inches in diameter.
• The detector port views the sphere wall directly above or below the entrance port so that highly divergent sources can be input without effecting measurement accuracy.
• A detector adapter accommodates filters up to 0.5 inches.
• Ability to mount either two detectors or one detector and a spectrometer to the detector port.
• No baffle is necessary enabling a better level of light integration.

The size and number of baffles should be minimized since baffles can cause certain inaccuracies because the integrating sphere is no longer a perfect sphere. It is also important to ensure baffles are strategically placed to maximize their usefulness while minimizing their impact on the integration of light.

1. Any filter-based photometer is only accurate when measuring a source similar to that with which it was calibrated.
2. The reference source is usually an illuminant A incandescent lamp.

Different spatial distributions of luminous intensity found in LEDs show the considerable variety that can be found and the associated difficulties of defining a uniform method of measurement and characterization.

1. Spatial Non-uniformity
2. Spectral mismatch between standard and test lamp
3. Near-field absorption
4. Photometer Temperature
5. Effects on heating of the sphere wall coating
6. Length of lamp

The considerations for integrating sphere design are:
1. The placement of the ports and baffles so that the total integration of light is not interrupted.
2. Sphere Proportions – the surface area should not be interrupted by more than 5% of the surface area.
3. Correction for Substitution Error.

Mechanical Tolerance
- Chips of different sizes, types, and geometries
- The chips are housed in complex structures to maximize effective intensity.
- Cup structures may be designed to reflect side emissions
- Lens designs to alter color, spatial distribution, and/or spectral distribution
Thermal Management (ambient and device temperature)
- ~ 0.1 to 0.2 nm/oC depending on LED type

The absorption of an LED source is one of two key errors commonly encountered when measuring total flux:
1. Sample absorption error when testing physically different sources placed within a sphere
2. Spectral mismatch between standard lamp and LED when using a photometer

To correct for this the sphere is calibrated by placing a lamp of known luminous flux or spectral radiant flux within the sphere, called a reference lamp. A reference lamp is typically a small, low power incandescent lamp. The test source can be anything. Then an auxiliary lamp is used to correct for the absorption error found with the tested source.

A spectroradiometer is made up of a detector and a grating monochromator. As with filter-based photometers & colorimeters, the input optics define the collection geometry. The spectroradiometer can be scanning or array based and measures spectral power distribution in small ?? hence reduced photopic errors. The photometric and colorimetric quantities are calculated from raw spectral power data (usually by software – thankfully!).

Since LED lenses create a dependence of measurement collection angle and measurements area, almost any value can be obtained, depending on how the conditions of the measurement are prescribed. TC 2-58 was designed to prepare a technical report setting out recommendations for measurement of the radiance and luminance of LEDs, taking particular account of the specific requirements for evaluation of photobiological safety.

Scanning spectroradiometer are slow but more accurate.

Array spectroradiometers are (much) faster but can be less accurate.

Tip: Choose an array-based spectroradiometer with =2nm spectral resolution and <0.1% stray light performance for acceptable accuracy with LEDs or other narrow band sources.

When the sphere wall is perfectly diffuse (a.k.a. Lambertian), then light reflected from any point in the sphere will be distributed over the sphere with perfect uniformity so that the true integration of the light takes place.

This was a revision of the CIE 127 LED measurement standard. The CIE Publication 127 was prepared several years ago to define recommended geometries for luminous flux and intensity measurements on LEDs.

Mission of TC 2-45: To revise Publication CIE 127-1997 to include improved definitions of quantities and methods of measurement for total flux and partial flux of LEDs and to reevaluate other parts including spectral and color measurements of LEDs.

This revision specifically excludes radiance measurements.

A baffle protects the detector from direct illumination from the light source so that the detector will only sense the integrated light coming from the sphere wall. The baffle also protects the detector from viewing the area on the sphere wall that the light source is directly illuminating.

The present color rendering system gives a poor rating for white LEDs yet the color appearance of white LEDs is better than color rendering index would suggest. This is a potential barrier to introduction of white LEDs into main stream applications so TC 1-62 was established to investigate, by visual experiments, color rendering properties of white LED light sources and to test the applicability of the CIE color rendering index to white LEDs.

The TC 2-46 CIE/ISO Standards on LED Intensity Measurements mission was to prepare a CIE/ISO Standard on the measurement of LED intensity measurements based on CIE Pub. 127.

TC 2-50 Measurements of LED clusters and arrays mission is to:
1. produce a technical report for measurement of the optical properties of visible LED clusters and arrays,
2. to derive optical quantities for large area arrays
3. and give recommendations for measurement methods and conditions.

LEDs are being increasingly used in clusters and arrays, for signalling and indication purposes, display and accent lighting, and for white light applications.

Special features of LEDs, such as pixilation, directionality, spectral characteristics, and pulsed operation, coupled with a lack of guidance regarding measurement methods and performance specification, make measurements difficult. Specifications generally established only for ‘traditional’ sources.

A: The design must resolve XYZ tristimulus functions, hence 3/4 detectors with tristimulus response filters (RGB or R1+R2GB). Accuracy is strongly driven by tristimulus response matching which is a similar problem with LEDs as photometers.

For the highest accuracy, make color measurements spectroradiometrically -- Y channel (green) = CIE V(?).

A: The larger the sphere, the better the geometric performance. Larger spheres offer better spatial integration. A sphere should be 10x the lamp size, if possible, or 2x for linear light sources. For example, Yoshi Ohno at NIST uses a Labsphere custom 2-meter tiled Spectralon® sphere to realize the US standard of luminous flux.

The smaller the sphere, the higher the signal level. Smaller spheres may be used for small sources, such as individual LEDs.

Sphere size is also determined by:
• Number of ports
• Size of ports
• Radiance level required at the exit port

As a rule of thumb, no more than 5% of the sphere surface area should be consumed by port openings.

The material choice depends mainly on:
• Spectral requirements
• Operating Environment

Go to the coating and reflectance chart to decide which coating would be best for you.

The simplest way is to use a system that has multiple lamps, sometimes of various powers. Lamps are activated to produce the desired output level. Where more resolution is required, an external lamp with a variable component is used. Variability of the light is achieved using filters or a variable shutter. Controlling output level by adjusting the current to the lamps is not recommended, since large spectral shifts will occur.

In visual applications, a photopic detector is sufficient. The reading from a “broadband” measurement, such as an unfiltered detector, depends on the spectrum of the source. Since the spectrum is stable, a simple detector provides adequate monitoring. Of course, a detector whose responsivity is stable over time is required for accurate calibration. In some cases, the detector must be thermally controlled so that its responsivity does not change.

Uniform Sources of IRRADIANCE can be used to back illuminate a printed or etched image such as photographic film for image digitization or resolution targets for MTF testing.

The source may also be used for testing a non-imaging device such as a CCD or similar array detector. The device being tested is often placed co-axial with the port, but at some distance away. When used in this way, the two important quantities to be determined are the axial irradiance at the center of the object as well as the irradiance at the off-axis edge.

RADIANCE Uniform Sources can be used to test and calibrate digital cameras, remote-sensing systems, and other electronic imagers.

An integrating sphere provides the best means of characterizing and calibrating the response uniformity of an IMAGING system. Integrating sphere uniform sources significantly out-perform alternatives such as reflective diffuser targets.

Labsphere’s line of Uniform Source spheres and systems are specifically designed for such applications, and are readily adaptable to provide variable levels of radiance, and even variable correlated color temperature, without affecting the uniformity of the scene presented to the system under test.

A uniform source system is an excellent tool to measure the responsivity of an array (NON-IMAGING system). A uniform source can provide a known amount of illumination. When the illumination level is varied and the array’s response measured, the responsivity, linearity, and dynamic range can be characterized. By introducing narrowband light of various wavelengths, the spectral response can also be measured.

A integrating sphere uniform source is also very useful for measuring a focal plane array’s photon transfer curve. By varying the level of input illumination, one can measure the noise in the array and determine the sources of that noise: noise "floor" under low photon flux conditions; shot noise as illuminance increases: and FPN at higher illuminance. This technique also gives the dynamic range of the array, including its associated readout electronics.

Uniform Sources typically include:
1. Integrating Sphere
2. Port Accessories
3. Lamps
4. Power Supplies
5. Variable Attenuators
6. Detectors
7. SC-5500
8. System Control Software

A good design guideline is that the sphere diameter should be at least three times the diameter of the exit port. Any smaller and uniformity will be compromised. Much larger, and many more lamps will be required with very little payoff in uniformity. To achieve a certain output level, the input power varies as the square of the sphere diameter, when all other parameters are held constant.

Radiance is the flux density leaving a radiant surface as viewed from a distance away from the surface. A Lambertian surface features a radiance that is perfectly diffuse, independent of viewing angle.

Irradiance is the flux density falling on a surface and is measured at the plane of the surface. Integrating sphere sources are most often used to test an imaging system. The desired effect is uniform radiance within the field-of-view of the system under test.

The material depends not only on the spectral requirements, but on the operating environment. For example, some diffuse coatings are more robust than others when used in humid environments. Damage thresholds in high energy applications must also be considered.

Coatings and materials used in integrating spheres have reflectances between 95% and 99%. When a perfect diffuse reflector is illuminated with uniform irradiance it behaves as a perfect diffuse source — a Lambertian source.

Labsphere’s Spectralon®, Spectraflect®, Duraflect® and Infragold®
coatings provide excellent Lambertian properties.

Just like other areas of spectroscopy, reflectance spectroscopy is applicable in a wide variety of industries. Some examples are life sciences, pharmaceuticals, textiles, coatings, and food.

A center mount sample holder allows a sample to be located in the middle of the integrating sphere of a reflectance measurement system. The sample can be rotated in order to measure the change in reflectance of the sample as a function of angle.

Most spectrophotometers and spectrometers are designed to measure transmittance or absorbance. A reflectance spectroscopy accessory allows you to use your existing spectrophotometer or spectrometer to make reflectance measurements of a wide variety of sample types.