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Q: How is CIELAB color space computed?
A: Typically, CIELAB is computed with some reference illuminant -- e.g., A, D65, or D75. These illuminants are normalized to a value of 100 at 560 nm. When doing a relative color measurement, the result is usually referenced to these illuminants:
That is, for reflection, transmission, and relative irradiance, it can be expected that the Y values will be approximately in the range 0-100, which translates to an L* value in that same range when computing CIELAB.
Absolute irradiance, however, is completely different.
In absolute irradiance, the spectrum is not normalized. That’s kind of the point — in the modes described above, the spectrum is always relative to something: a reference illuminant, a lamp that is assumed to be a blackbody source and so on, and by being normalized they are bound to some convenient limit (like 100). Absolute irradiance is not normalized to anything: the magnitude is computed in absolute terms against standard physical constants. The spectrum that results from an absolute irradiance calculation describes the light source being measured, and the magnitude is bound only by the possible intensity of that light source and the limits of the instrument. If you were to accurately measure the surface of a supernova with absolute irradiance, the total power (the spectral magnitude) would literally be astronomical. There is no bound of 100 anywhere. In other words, “100 for the brightest white” is all relative unless you mean “the brightest white that is possible in the entire universe”.
The fundamental problem is that the L* value in CIELAB is not an appropriate measure of absolute brightness. It might be useful for a reflective, transmissive, or possibly even relative irradiance measure of brightness, since these are relative. For absolute brightness, there is a whole other set of photometry measurements, including lumens, lux, candela, and luminance. These have no upper bound and are expressed (as absolute irradiance is) in standard physical units.
The reason that L* is being computed as being quite small is because the total amount of power (uWatts/cm^2/nm) being measured from the user’s monitor is quite small. Monitors do not produce much photonic power, which is what absolute irradiance is measuring.
If you were to do a relative irradiance measurement instead, using a white area of the screen as the reference, you might find that their L* value is in better agreement with what you expect. This would be true only because you would be making a measurement relative to something else; in this case, “the brightest white” that the display can generate, which becomes the definition of L* = 100 for the duration of that measurement.
If you wanted to see whether the absolute irradiance calibration was credible, you could try to measure daylight and compare that to the average solar irradiance for their part of the world. The average “insolation” (solar irradiance) reaching the surface is about 250 W/m^2, but this varies with latitude, time of day, and season. You may be able to look at the integrated power being measured across the absolute irradiance spectrum and see how it compares (though some unit conversion may be required).
Any time that a comparison is done between computer color spaces (typically RGB) and colorimetry based on models of human perception (typically XYZ, which leads to things like CIELAB) there is significant risk of error. RGB is usually nicely bound to 0-255 for each of its values. Conversion matrices have been created (with implicit and usually unstated reference illuminants and observers) to try to map from computer color spaces to human-model color spaces. These conversions are imperfect, and there are many cases where converting from one color space to another this way results in invalid color values. For example, the human-model color spaces cover a much larger range of colors and intensities than RGB can represent. Any time that a computer tries to convert between its own RGB colors into a human-model color space (such as CIELAB), one must be suspicious of the result. For instance, the “Digital Color Meter” application provided with some desktop computers is not necessarily authoritative on color measurement. It might be okay for relative color measurements (if the reference and dark spectra are taken as the brightest and dimmest the monitor can render — note too that “black” on a monitor will often still allow some of the backlight through), but it is not meaningful to use it for, or in comparison to, an absolute measurement of intensity.
Q: How do I determine divergence in single lens systems?
A: The divergence (a) of a beam focused using a single lens is tan(a) = d/f where f is the focal length of the lens and d is the aperture or fiber diameter.
Q: In probe-based Raman setups, what is the focal spot at the sample site?
A: For our RIP-RPB general-purpose Raman probes with 105 µm excitation and 200 µm collection fibers, the theoretical spot size is 158 µm with a 7.5 mm focal length or working distance. The depth of field is approximately 2.2 mm.
With the optional 5 mm working distance for the RIP-RPS stainless steel probes, the theoretical spot size is 105 µm with an approximate depth of field of 1.0 mm. Note that the actual spot size is about 20% higher due to spherical aberration. Additionally, the spot size highly depends on the sample properties. A reflective sample will have a spot size like the theoretical prediction and a shorter depth of field, whereas a transparent sample will have a larger spot size and greater depth of field.
Q: How can I reduce noise in my spectra?
A: You have several options available to you to minimize or eliminate the noise in your spectra:
Q: How do I calculate PAR (Photosynthetically Active Radiation)?
A: The amount of energy per photon at a given wavelength is computed as:
E = hc/L
E is the total energy measured in Joules
h is the Planck constant (6.626E-34 J*s)
c is the speed of light (299792458 m/s)
L is the wavelength of the photon measured in meters
Given that absolute irradiance is in units of uWatts/cm^2/nm, the conversion is as follows.
Q: What is the best sampling device for my Raman measurements?
A: Raman spectroscopy can be performed on samples ranging from liquids and gels to powders, capsules, and large objects. A fiber optic probe assembly is the sampling optic most often used in a modular Raman spectroscopy system. It consists of a fiber bundle to route excitation light from the laser to the sample and collect Raman scattered light. Light at the laser wavelength (Rayleigh scattering) is rejected on the path to the spectrometer using a dichroic filter to avoid saturating the detector.
We offer a range of fiber optic probes for Raman spectroscopy with 532, 638, 785 and 1064 nm excitation. The RIP-series probes include models for laboratory, industrial and environmental applications with SMA 905 or FC connectors. The process probes are rated for use at up to 200 °C and 1500 psi, while the immersion probes go up to 500 °C and 3000 psi. Options include stainless steel and Hastelloy C housings and sapphire windows.
These probes mate well with the RM-LQ-SHS and OOA-RAMAN-SH sample holders, which are compatible with a variety of cuvette sizes and vials. RM-LQ-SHS has a magnetic cover, making it easy to load and unload samples, block ambient light, and improve the accuracy of Raman measurements. OOA-RAMAN-SH has a positioning screw that allows the user to adjust the distance between the probe and the sample simply and accurately to optimize the Raman signal while blocking ambient light with a cap. The OOA-HOLDER-RFA has many of the same features, but is designed as a multipurpose sampling fixture for Raman, fluorescence, absorbance and reflection measurements, with accessories for each technique. In addition, the RM-SERS-SHS holder is ideal for surface enhanced Raman spectroscopy (SERS) substrates that comprise nanoparticle chemistries on slides. This holder accommodates standard glass SERS slides and connects to a Raman probe. The RM-SERS-SHS provides accurate positioning for the Raman measurement, avoids the influence of ambient light, and improves the accuracy of the Raman measurement.
Q: How do I determine the spot size of my sampling setup?
A: Let’s use the drawing below to demonstrate how to get to your answer. In this example, the distance (“X” on the drawing) is 3 inches.
First, we calculate the half angle (12.7°) of the fiber, which is based on its numerical aperture and index of refraction in air. The NA of our optical fibers and probes is 0.22 +/- 0.02 and yields an acceptable angle of 25.4° in air.
Numerical Aperture formula
Na = Л Sin(θ)
θ = Sin-1 (Na/ Л)
θ = Sin-1 (0.22/ 1.00033) = 12.7°
Given that …
Numerical Aperture = 0.22
Л (Index of refraction in air) = 1.00033
X (Distance) = 3 inches
h (Height, or the distance from the tip of the fiber or probe to the sample surface) =?
θ (Half Acceptance Angle) =?
Sin(θ) = (h/X)
h = X Sin(θ)
Spot Size = 2h
Spot Size = 2(X Sin(θ))
= 2(3 Sin (12.7°))
Spot Size = 1.32”
Q: What is the relationship between spectrometer slit size and optical resolution?
A: Optical resolution of a monochromatic source -- measured as Full Width Half Maximum (FWHM) -- depends on the groove density (mm-1) of the grating and the diameter of the entrance optics (optical fiber or slit). In configuring your spectrometer, consider two important trade-offs:
The slit is the small opening that allows light to enter the spectrometer. The slit width is inversely related to the optical resolution of the spectrometer such that smaller slit widths will produce greater resolution. A smaller slit, however, allows less light into the spectrometer. For most Ocean Insight spectrometers, the slit height is 1000 μm and only the width varies (from 5 μm up to 200 μm). For spectrometers with no slit, the diameter of the optical fiber limits the amount of light entering the system and performs the function of the slit.