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Technical Documents

Ocean Optics provides technical information and applications know-how to help you evaluate, operate and optimize our products for your measurement challenges. Our interactive Range & Resolution Calculator is a useful tool for determining anticipated spectrometer performance.

Getting Started visual guide, operating instructions, engineering-level specifications and material related to spectrometer performance are available for you. For further assistance, please contact an Ocean Optics representative.

Light Sources



Radiometrically Calibrated Light Sources

Wavelength Calibration Light Sources
Integrated Systems

Note: The following information applies to Ocean Optics spectrometers only.

Interactive Optical Benches

View Interactive Optical Benches for:

How to Calculate Optical Resolution

Optical resolution of a spectrometer, 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).

Formula for Calculating Optical Resolution

1. Determine the spectral range of the grating. Look at the grating charts and note the value in the spectral range column in the chart. For example, Grating #3 has a spectral range of ~650 nm. Please note that the spectral range can vary by starting wavelength, which is why 650 nm is an approximation.

2. Determine the number of pixel elements in the spectrometer’s detector. For a USB2000+ spectrometer, the number is 2048. Divide the grating spectral range by the number of pixel elements in the detector. This is your dispersion value. For our example, 650 nm/2048 pixels = 0.32 nm/pixel.

3. Choose a slit width. Each slit has a pixel resolution value. For a USB2000+ with a 10 µm slit, this value is ~3.2 pixels.

Optical Resolution = Dispersion x Pixel resolution

In our example, the dispersion equals to 0.32 nm/pixel. Multiply it by 3.2 of pixel resolution, and you get that the optical resolution of USB2000+ spectrometer with 10 µm slit is 1.02 nm.

Beer's Law (Beer-Lambert Law)

Beer-Lambert Law, more commonly known as Beer’s Law, states that the optical absorbance of a chromophore in a transparent solvent varies linearly with both the sample cell pathlength and the chromophore concentration. Beer’s Law is the simple solution to the more general description of Maxwell’s far-field equations describing the interaction of light with matter. In practice, Beer’s Law is accurate enough for a range of chromophores, solvents and concentrations, and is a widely used relationship in quantitative spectroscopy.

Absorbance is measured in a spectrophotometer by passing a collimated beam of light at wavelength λ through a plane parallel slab of material that is normal to the beam. For liquids, the sample is held in an optically flat, transparent container called a cuvette. Absorbance (Aλ) is calculated from the ratio of light energy passing through the sample (I0) to the energy that is incident on the sample (I):

Aλ = -log (I/I0)

Beer’s Law follows:

Aλ = ελbc

ελ = molar absorptivity or extinction coefficient of the chromophore at wavelength λ (the optical density of a 1-cm thick sample of a 1 M solution). ελ is a property of the material and the solvent.

b = sample pathlength in centimeters

c = concentration of the compound in the sample, in molarity (mol L-1)

In an absorbance experiment, light is attenuated not only by the chromophore, but also by reflections from the interface between air and the sample, the sample and the cuvette, and absorbance by the solvent. These factors can be quantified separately, but are often removed by defining I0 as the light passing through a sample “blank” or “baseline” or reference sample (for example, a cuvette filled with solvent but zero concentration of the chromophore is used as the blank).

Many factors can affect the validity of Beer’s Law. It is usual to check for the linearity of Beer’s Law for a chromophore by measuring the absorbance of a series of standards. This “calibration” can also remove errors in the experiment, the equipment, and the batch of reagents (such as cuvettes of unknown pathlength).

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