Technical Guideintegrating Sphere uniform light Source applications

Uniform Light Source Applications

TABLE OF CONTENTS

1.0 Applications 3 1.1 Focal-Plane Arrays 3 1.2 Imagers 4 1.3 Sensitometry 4

2.0 System Design Considerations 5 2.1 Sphere Size 5 2.2 Uniformity 5 2.3 Spectrum 5 2.4 Light Level 5 2.5MechanicalConfiguration 6 2.6Monitoring 6

3.0Frequently-AskedQuestions 6

4.0 Calibration 7 4.1 Luminance 7 4.2 Spectral Radiance 7 4.3 Irradiance 7 4.4 Uniformity 8

5.0 Custom Labsphere Uniform Source Systems 9 1.6mUniformSource 9 2m High Power Uniform Calibration Source 9 Sensitometer Source 9 Multispectral Uniform Source 10 Projected Uniform Source 10 Appendix AIntegrating Sphere Theory 11-12 References 13

1

Uniform Light Source Applications

INTRODUCTION

Uniform source systems provide uniform radiance orirradiance. Their applications include focal-plane arrayor complete camera testing, pixel gain normalization,photographic sensitometry, and remote-observation systemcalibration. Sources are used in the ultraviolet, visible, andinfrared regions of the spectrum. Most uniform sources useintegrating spheres, while some use lasers, lamps, and other sources alone. Sphere systems are highly lambertian and show very uniform radiance distribution. This Techguideexamines some of these applications, discusses howLabsphere calibrates uniform sources, and providesexamples of custom uniform light systems we have de-signed.

Labspheres design engineers are ready to help you withyour uniform source needs. After you have had a chanceto read this booklet, call us at (603) 927-4266 and one ofour technical representatives will assist you with yourapplication.

1.0 TypICal applICaTIONs

1.1 Focal-plane arrays

Focal-plane arrays (FPA) aremulti-element photosensitivedetectors used in electronicimaging. Their applicationsrange from inexpensive videoand still cameras in theconsumer market to advancedscientific imagers for spaceborneremote sensing. Thereare various types of FPAs, such as charge-coupled devices(CCD), charge-injection devices (CID), complementarymetal-oxide semiconductors (CMOS), and photodiodearrays (PDA). They can be linear or two-dimensional andtheir size is usually not more than several centimeters.Once fabricated and packaged, the devices can suffer fromsome degree of non-uniform gain and offset coefficients.When exposed to an equal irradiance of light, each pixelin the array does not produce an identical electrical signal.Photoresponse Nonuniformity (PRNU) is due to differences in responsivity (gain) among the pixels in the array and Fixed-Pattern-Noise (FPN) is due to variation in dark current (offset). In the presence of gain and offset variations the device produces images that have features

that do not exist in the original object but are impartedon the image by the array. In other words, a picture of auniform field is not uniform. This type of application is anirradiance application.

Offset normalization is simply performed by providing zeroirradiance on the array and setting the output signal of eachpixel to zero. During gain normalization, a uniform sourceproduces irradiance on the array that is equal at each pixel.The gain, or responsivity, of each pixel is set so that eachpixel produces an equal electrical signal. Linearity can bemeasured by irradiating the array with varying light levelsand measuring the signal produced.

Uniform irradiance may be produced by placing the FPA atthe exit port of a uniform source sphere. At the exit port, thearray will be irradiated from all directions. If a limited field isdesired, the array can be located some distance from the exit port. This distance, and the size of the exit port must bechosen to provide the required field angle and to ensureadequate uniformity. (See Appendix A page 10.)

It is useful to know the responsivity of an array. A uniformsource can provide a known amount of illumination. Whenthe illumination level is varied and the arrays responsemeasured, the responsivity, linearity, and dynamic range can be characterized. By introducing narrowband light of various wavelengths, the spectral response can also be measured.

A uniform source system is an excellent tool to measure anarrays photon transfer curve. By varying the level of inputillumination, one can measure the noise in the array anddetermine the sources of that noise: noise floor under lowphoton flux conditions; shot noise as illuminance increases:and FPN at higher illuminance. This technique also givesthe dynamic range of the array, including its associatedreadout electronics.1

1.2 Imagers

Digital cameras, remote-sensingsystems, and other electronicimagers must be normalized inmuch the same way as bare FPAs.However, one more element isintroduced that must beaccounted for the opticalimaging system.

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Uniform Light Source Applications

Imaging systems, whether they be refractive, reflective, orboth (catadioptric), suffer from irradiance that varies withfield angle. The most common variation is the cosine-fourthlaw.2 (see Figure 1) A procedure similar to that described forsimple FPAs will correct for cosine-fourth irradiance falloffand other sources of irradiance variation in the image. Thisapplication is a radiance application.

1.3 sensitometry

Manufacturers of silver-halide photographic materials mustperform quality-control tests on their products. The lightsource irradiating the target must be uniform over a largearea. The irradiation must be controllable in level and, insome cases, spectral content. Complete systems must alsoincorporate a manufacturers process-control data collectionrequirements, such as logging data and providing statisticalinformation. This application is an irradiance application.

In some cases, a uniform source sphere is the best choice for sensitometry. It can provide optical power efficiency andexcellent uniformity. A sphere also allows the designer tomix several sources to provide simultaneously a multi-source spectrum and uniform irradiance. In other applica-tions, a non-sphere based system is best. The difference is in the geometry. If diffuse illumination is required, then we place the sample at the spheres exit port. When directionalillumination is required, a beam is projected at the target.

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Uniform Light Source Applications

2.0 sysTem DesIgN CONsIDeRaTIONs

There are several system design requirements below that are common to most uniform source applications. These are the parameters that the designer must take into account when formulating the sources specifications.

2.1 sphere size

The size of the source is often specified explicitly in imagingapplications. The diameter of the exit port of a sphere, forexample, may be based on the object size, aperture, and field of view of the camera under test. In irradiation applica-tions, the size of the source may be designed based on parameters such as the irradiated area and distance from the source to the target. The size of the integrating sphere is then based on the required size of the exit port. The larger the sphere with respect to the exit port, the greater the uniformity, all other things being equal. However, the power required in the sphere to produce a particular radiance or ir-radiance increases with sphere size. Sometimes, designers must make compromises on the overall size of the system based on available volume for the system, shipping and storage constraints, and cost.

2.2 Uniformity

A uniform source system will provide radiance andirradiance uniformity. As important as the uniformity value,is the method by which uniformity is measured. Commonmethods include mapping the radiance of the source usingan imaging system, or mapping the irradiance at a targetplane using a simple detector with a wide field of view.Irradiance uniformity when measured a distance from thesource, depends on geometrical theory as well as theradiance uniformity of the source (see Appendix). In someapplications, especially remote sensing, angular uniformitymapping is also required.

2.3 spectrum

The spectral distribution of the light coming from a sphereis based on the light source and the reflectance of the sphere material. Spectral requirements influence the choice of light sources and reflectance materials. Most uniform source systems incorporate tungsten-halogen lamps. By placing the lamps outside of the sphere, filters allow the spectrum to be tailored to the desired shape.

Spheres allow one important feature that is not availableusing other methods: multiple sources can be mixed in thesphere to produce uniform output. For example, severalnarrowband sources, such as lasers, LEDs, or red, green,and blue sources, can be input to the sphere. Light from thevarious sources will be integrated and the resulting outputwill be the combination of the relative distributions, as wellas the effects of the sphere wall reflectance.

2.4 light level

The quantity of radiance depends on the size of the sphere,the reflectance of the sphere wall, and the amount of sourcepower input to the sphere. Irradiance at a distance from theexit port depends on the radiance of the exit port, its size,the distance from the sphere to the target, and the size ofthe target (see Appendix).

It is also important to consider varying the light outputlevel. The radiance of a sphere with multiple sources, forexample, will vary with the number and power of thesources that are activated. Resolution in the adjustment issometimes critical.