Spectrophotometry
Spectrophotometry is a technique that determines how materials interact with light as a function of wavelength and other attributes of both the material and the light. In contrast to spectrofluorometry, the dispersion performed by the monochromator or interferometer takes place either between the source and the sample or between the sample and the detector, not in both locations. We describe each of these properties and the range of these properties that ODA can address with its four major spectrophotometers.
| Instrument | Nanometer - nm Range | Micron - µm Range | Wavenumber - 1/cm Range | |||
|---|---|---|---|---|---|---|
| Agilent Cary 7000 Dispersive UV-Vis-NIR 5° - 80° Angles of Incidence & Exitence |
350 | 2,500 | 0.35 | 2.5 | 28,572 | 4,000 |
| Agilent Cary 5000 Dispersive UV-Vis-NIR Normal/Near Normal AOI |
200 | 3,000 | 0.20 | 3.0 | 50,000 | 3,030 |
| Perkin-Elmer 983G Dispersive IR Normal/Near Normal AOI |
2,000 | 55,600 | 2.0 | 55.6 | 5,000 | 180 |
| ThermoScientific iS10 Fourier Transform IR Integrating Sphere |
2,000 | 25,000 | 2.0 | 25.0 | 5,000 | 400 |
| Instrument | Layout | Sources | Sample Compartment | Detectors |
|---|---|---|---|---|
| Agilent Cary 7000 Dispersive UV-Vis-NIR |
Sources - doublemonochromator - sample - detectors | Tungsten-halogen/deuterium | Double-beam, focus in center | Si/InGaAs |
| Agilent Cary 5000 Dispersive UV-Vis-NIR |
Sources - doublemonochromator - sample - detectors | Tungsten-halogen/deuterium | Double-beam, focus in center | PMT, cooled PbS |
| Perkin-Elmer 983G Dispersive IR |
Source-sample-monochromator - detector | Globar™ | Double-beam converging | Thermocouple |
| ThermoScientific iS10 Fourier Transform IR |
Sources- interfero-meter - samples- detector | Globar™ | Single-beam-collimated | DTGS |
| Transmittance | Angle | Reflectance | Angle | ||
|---|---|---|---|---|---|
| Incident | Exitent | Incident | Exitent | ||
| Normal Direct [Specular] Absolute | 0° | 0° | Near-Normal Specular Absolute | 7° | 7° |
| Variable Angle Direct [Specular] Absolute | 0 - 85° | 0 - 85° | Variable Angle Specular Absolute | 5° - 85° | 5° - 85° |
| Normal Total Absolute | 0° | 2π sr Forward | Forward Near Normal Total Referenced | 7° | 2π sr Backward |
| Variable Angle Total Absolute | 0° - 75° | 2π sr Forward | Variable Angle Total Referenced | 15° - 75° | 2π sr Backward |
Polarization
The incident beams from all four instruments are somewhat polarized due to the presence of gratings and non-normal reflections in the optical train. The parallel - p - and perpendicular - s - polarizations can be specified and isolated with polarizers over the 0.35 < λ < 2500 nm spectral range of the two Agilent SPMs.
ODA does not own IR polarizers for λ > 3000 nm, so polarization cannot be controlled in this wavelength range.
Spectrofluorometry
Spectrofluorometry is commonly defined as a type of spectrophotometry deals with fluorescence, the phenomenon in which light incident on a material at a given wavelength, usually in the ultraviolet or visible spectral range, causes that material to radiate light at longer, less energetic wavelengths. To observe this effect, spectrofluorometers have dispersive optics, usually grating monochromators, both between the source and the sample for the incident light and between the sample and the detector for the exitent light. This permits the identification of the wavelengths that excite the sample and the wavelengths at which that excitation generates the fluorescence. Obviously, fluorescence can produce spurious signals in an optical system, so that fluorescent optics and their excitation and emission spectra should be recognized and accounted for. On the other hand, the fluorescence phenomenon is exploited by technologies ranging from household cleaning products to advanced biotechnology. ODA operates an Agilent Eclipse instrument to make these measurements.
| Instrument | Nanometer - nm Range | Micron - µm Range | Wavenumber - 1/cm Range | |||
|---|---|---|---|---|---|---|
| Agilent Eclipse | 190 | 1,100 | 0.19 | 1.10 | 52,632 | 9,091 |
| Instrument | Layout | Sources | Sample Compartment | Detectors |
|---|---|---|---|---|
| Agilent Eclipse | Source - monochromator - sample - monochromator - detector | Pulsed xenon arc lamp | Single-beam, focused | Integrating PMT |
Orientation
The instrument accommodates cuvettes with a standard 1 cm square cross-section. These are illuminated normally on one face; the emitted light from an adjacent face, that is, at a 90° angle from the incident beam, is detected. It can also accommodate solid samples. For the solid sample attachment for the Agilent Eclipse the excitation beam is incident at 15°; the exitent beam is therefore at 75°.
Sample Properties
Solid Samples
Flat Specular Optical Elements and Other Materials
These materials are measured with direct transmittance sample holders or with the specular reflectance accessories. Ideally, samples have diameters or square sides of 2.5 to 7.5 cm [1 to 3 inches], but samples with dimensions as small as 1 mm [0.040 inch] and as large as 0.75 m [30 inches] can be accommodated with special jigs and holders.
Curved Specular or Rough Optical Elements, Powders, and other Materials
These materials are usually measured at the transmittance or reflectance port of the appropriate integrating sphere. Ideally, samples have diameters or square sides of 2.5 to 7.5 cm [1 to 3 inches], but samples with dimensions as small as 5 mm [0.040 inch] and as large as 0.75 m [30 inches] can be accommodated with special jigs and holders. If both reflectance and transmittance must be measured at the same time ["transflectance"], the sample can be measured at the center of the sphere ["Edwards" mode], but the dimensions must not exceed 2.5 cm.
For the Cary 500E [UV-Vis-NIR], two 150-mm diameter integrating spheres, both made by Labsphere, are available.
- The first is known as a "side-looking" sphere; the entrance and exit ports for the sample and reference beams are arrayed along the horizontally oriented "equator" of the sphere. The access port is at the "top" pole, and the detector port is at the "bottom" pole. This means that samples are mounted "vertically" on the sides of the sphere.
- For T measurements, the sample is mounted in front of the entrance port. The specular portion of the can be included [total T] if the exit port on the opposite side is covered with a Spectralon standard identical to that of the sphere walls, or it can be excluded [diffuse T] if the exit port is covered with a black standard or light trap. Samples up to 10 cm across can be accommodated.
- For R measurements, the specular portion of the beam can be included [total reflectance] or excluded [diffuse reflectance] , depending on whether a small portion of the sphere wall - which is on a removable plug - in the specular direction remains in place or is removed. Samples up to about two meters in maximum dimension can be accommodated.
- For powder R measurements with this sphere, ODA employs a powder cell made by Avian Technologies with a quartz window and a Spectralon™ cavity; other arrangements can be made if necessary.
- The second is known as a "down-looking" sphere; the entrance and exit ports for the sample beam are arrayed along the vertically oriented "equator" of the sphere. The sample beam can be switched between an entrance port at the side of the sphere or another entrance port at the "top" pole of the sphere.
- For T measurements, the sample is placed vertically in front of the entrance port on the side of the sphere. Only total T measurements can be made, but samples up to 20 cm across can be accommodated, about double the capacity of the side-looking sphere.
- For R measurements, the sample is placed horizontally against the port at the "bottom" pole of the sphere, and that powders and liquids can be accommodated without the use of a cell. The beam is directed downward by a mirror outside the entrance port at the top of the sphere. As with the "side-looking sphere " the specular portion of the beam can be included [total R] or excluded [diffuse R] , depending on whether a small portion of the sphere wall - which is on a removable plug - in the specular direction remains in place or is removed.
A diffuse R accessory involving specular optics and a upward-looking powder holder is also available.
For the Nicolet 560 [IR], a 102 -mm diameter sphere of gold-plated aluminum [InfraGold™] is available. The entrance port for this sphere is on the side of the sphere, so transmittance samples are mounted vertically. Because this sphere employs a specular mirror at its center to deflect the beam along a vertically oriented great circle, reflectance samples can be mounted at the bottom port, permitting the measurement of liquid or powder samples.
There is no integrating sphere capability for the Perkin-Elmer 983G.
Liquid Samples
For the UV-Vis-NIR, ODA maintains a variety of UV quartz, glass, and plastic cuvettes with path lengths ranging from 1 to 50 mm, including matched pairs.
Optical Density
In general, for transmittances down to about 0.01= 1%, measurements are made with no reference beam attenuation in a linear, transmittance mode . For transmittances below 0.01 = 1%, attenuators are placed in the rear beam and the measurements are expressed as absorptance, the negative base 10 logarithm of the transmittance. The limits for the Cary 5000 UV-Vis-NIR SPM are about A = 6.3 in the UV-Vis ranges, and about 4.5 in the NIR range. The limit of the PE 983G IR SPM is about 3 over the MIR & FIR range.
Temperature & Humidity
Most measurements are made at ambient temperature [T]. However, ODA operates a cryostat that can measure transmittance [T] at T's down to about 85K, approaching liquid nitrogen's T of 77 K, For these measurements, a diameter of 2.5 cm is standard; other sizes will incur tooling charges. ODA has measured T of samples at up to 800 °C in heater blocks for special projects. Also, T measurements have been made for fixed levels of humidity in special cells for periods of up to six weeks. ODA also performs measurement before and after exposure to environmental tests to identify failure modes. ODA can subcontract these exposures if requested.
Microtopometry
Microtopometry describes the measurement of surface features, or topography, with a lateral resolution on the order of micrometers, or microns. Critical data for optical engineers, including:
- surface roughness, measured in terms of average roughness, root-mean-square, or rms, roughness, and peak-to-valley roughness, and
- step height, measured as the height above an averages rectangular substrate area,
can be provided in the form of false-color topographic maps or profiles. Such data can guide optical fabrication, substrate preparation, and thin-film development. Depending on the objective employed in the Micromap 512 Mirau inteferometer system described below, the map can cover an area roughly 1 mm square or 0.3 mm square; The horizontal resolution is roughly the wavelength of visible light; about 0.5 µm. The vertical resolution is as small as a few tenths of a nm, but the system can measure step heights and topography as large as 35 µm.