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Applications & Spectra

This applications journal is provided to give you some ideas on how to make optical sensing experiments. This is a somewhat random selection gathered from helping our clients deploy more than 100,000 spectrometers, phase fluorometers and other fiber optic sensing systems world wide. 

Please feel free to copy any of the spectra you see. Credit to Spectrecology would be appreciated, a link to our website would be even better!

If you need the actual data or help with the application please feel free to contact us.Here is a funny video that illustrates what we mean

Reflection Probe Fluorescence using 365nm Excitation

Fluorescence from solids or liquids is easy to measure with a fiber optic reflection probe. This versatile device has a bundle of fibers in a close packed 6 around 1 arrangement. The fibers are fixtured into a 1/4" diameter steel ferrule. The fibers are routed into 2 legs: the six outside fibers are the illumination leg and the inner fiber is the detection leg. In air the filed of view is ~25 degrees, in water its ~12 degrees. The sampled area of solids can be controlled by adjusting the distance from the probe to the surface. In liquids there is a sampled volume with the signal being the strongest from the nearest overlap region and diminishing as you move further way. In clear fluids the sampled volume may extend a few cm. In turbid fluids it will be much less. The best probe to use is one made from 600 um fibers, although 400 micron fibers will work as well.

the best fluorescence probe ever -- 6:1 configuration gives highest signal in liquids or on solids

The reflection probe is connected to our LLS-365 source for excitation. This is a powerful UV LED that can be used for a wide variety of fluorophores. The detection leg is connected to a USB4000-FL spectrometer. The spectrometer measures from ~350 - 1000nm with about a 5nm bandpass. It is set up for maximum sensitivity by using a 200 um slit and a collection lens on the detector. Here is a picture that shows the essentials. The reflection probe is in a holder to orient it at 45 degrees to the surface of a solid. This also keeps out room light.

deep UV LEDS and sensitive miniature spectrometer makes a great fluorometer system

Here are some sample spectra. The photopigments in plants are always quite interesting.  The main pigment is chlorophyll, but accessory pigments help harvest light and also give various colorations to the leaves. These are spectra taken in the green and yellow areas of the front surface, and the back surface. The green area has equal peaks at 690nm and 750nm, yellow has a strong peak at 690 and the back somewhat higher at 690nm.

spectrophotometer calibration procedure

Color matching and metamerisim

How can you repair a worn and scratched up leather couch? The color of the leather came from a dye that was mostly on the surface. Abrasion (mostly from chihuahuas!) scraped off the color and left a rough scattering kind of surface that mostly looked gray.

spectroscopic analysis of leather magic

 Of course the original pigemnts used when the couch were made are not available, not is there any kind of record of the composition of the dye.

Luckily we can match colors, by mixing pigments in such combinations that the mix appears to have the same color as the original substance, even though the intrinsic property of spectral reflectivity may be differnt.

The reason is the eye is essentially a 3 color sensor, and the net perception is a combination of the signals from these 3 color sensors. The sensors have broad response, so that all the color sensors can sense light across the visible spectrum, but their sensitivity is optimized for a particular color.

We sent an undamaged piece of leather to a firm called leather Magic in North Carolina. They belnded pigemnts to match the color values of the test piece.

after treatment with leather magicThe result (after some exhausting cleaning, prepping and coloring) was truly remarkable. The restored leather matched the leather sample exactly (as determined by eye).

The spectra analysis shows something differnt! In fact the reflection spectra of the new pigments and the old pigemnts are significantly different. As shown below, the new leather has a much higher NIR reflectivity, and a different shape in the red region. The two small peaks are in fact from two pigemnts that were belnded to match the color of the original single pigment.

The NIR spectra are also intersting from a heat budget perspective. When exposed to sunlight, the couch warms up. The higher NIR reflectivity might result in a cooler couch.

fluorescence spectrometers