Transmission & Absorbance

 

 

Beer-Lambert Law

Transmission measurements (%T) are often used to characterize optical materials, filters, thin film coatings, lenses and other optical components. Transmission is also used to measure the concentration of chemicals in liquid or gaseous samples. Absorbance (A) is a logarithmic transform that is linear with respect to concentration (C) of the absorbing species, the optical pathlength (l) and the extinction coefficient of the chemical (e)

A = eCl     or     C = A/el

The basic optical setup as shown above is to send a collimated beam of light normal to the surface a planar slab of sample. The detection is on the other side.  The light that is collected without the sample in this path is the reference (R), the light that gets through the sample is (S).  We also measure a background level or dark measurement (D).   Transmission (T) and Absorbance (A) are calculated at each wavelength as:

T = (S – D)/(R – D)          A = -log10(T)

The purpose of the probes and fixtures is to provide this basic sampling geometry. The following are a variety of absorbance and transmission measurement fixtures for a variety of applications.

 

Illuminated Cuvette Holders – FLAME CHEM

compact, simple, ready to systems for measuring absorbance of samples in 1cm cuvettes…

The direct attach VIS-NIR light source and built in cuvette holder bolts to the front of a FLAME spectrometer. Power and control is passed through the connector on the FLAME spectrometer and the unit becomes completely controlled through software.  The cuvette holder accepts all standard 1cm cuvettes (plastic or quartz styles). Outside dimensions of “1cm” cuvettes are actually 12.5 x 12.5 mm. Our beam height is 15mm. The VIS-NIR version pictured above is powered by the USB.

The UV-VIS version shown below has a miniature deuterium bulb and a tungsten halogen lamp. It can operate over the 200-1100nm range.

Spark and STS systems for absorbance

 

Fiber Optic Cuvette holders – CUV-ALL

 

The CUV-ALL-UV 4-way cuvette holder for 1 cm pathlength cuvettes has fiber optic couplings at each of four collimating lenses, which couple to optical fibers to illuminate, excite or read the sample. When used with Ocean Optics spectrometers and light sources, the CUV-ALL-UV measures absorbance, fluorescence, scattering or any combination of these optical phenomena. Optical accessories are available to optimize signal reflection, accommodate linear variable filters or block ambient light.

CUV-UV transmission absorbance cell holder

The CUV-UV cell holder is equipped with 2 fiber optic collimating lenses (74-UV) mounted in a standard 1cm cuvette holder. The holder has a spring loaded pin for precise positioning of the cuvette. It also features a filter slot for adding spectral bandpass or other optical filters. The AL base has water channels for connection to a circulating temperature controlled water bath. The 74-UV lenses can be removed for use in other fixtures.

CUV-FL-DA

The CUV-FL-DA cuvette holder attaches to Ocean Optics light sources and couples via optical fiber to our spectrometers, creating a small-footprint system for fluorescence and relative absorbance experiments. The CUV-FL-DA is optimized for the 200-2000 nm range, holds 1 cm pathlength square cuvettes and eliminates the need for a fiber from the excitation or illumination source. A few simple adjustments allow you to use the CUV-FL-DA for absorbance.The CUV-FL-DA is well suited for fluorescence, thanks to UV-enhanced, aluminum-coated mirrors that enhance signal reflection within the sample compartment. That increased signal reflection, combined with greater signal intensity resulting from direct attachment to the excitation source, provides CUV-FL-DA users with much greater fluorescence intensity than is possible with most cuvette holders.