what is a spectrophotometer?
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Spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color, or more specifically, the wavelength of light. There are many kinds of spectrophotometers. Among the most important distinctions used to classify them are the wavelengths they work with, the measurement techniques they use, how they acquire a spectrum, and the sources of intensity variation they are designed to measure. Other important features of spectrophotometers include the spectral bandwidth and linear range.\n\nFollow the link for more information.\n\nhttp://en.wikipedia.org/wiki/Spectrophotometer
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A non-invasive analytical technique with an infinitely broad range of applications in plant biotechnology and food processing. It is similar to MRI (Magnetic Resonance Imaging) in that it can be used for medical applications. Non-destructive analysis with this tool can be applied to a wide range of research challenges, including the determination of chemical structure, microimaging and plant metabolism studies.\npbi-ibp.nrc-cnrc.gc.ca/en/media/glossary.htm\n\nAn analytical technique devoted to the identification of the elements and the elucidation of atomic and molecular structure by measurement of the radiant energy absorbed or emitted by a substance in any of the wavelengths of the electromagnetic spectrum in response to excitation by an external energy source.\nwww.fire.org.uk/glossary.htm
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A spectrum is the derivative of broken light through a triangular shaped crystal, giving  as a result a spectrum or a rainbow of colors, and is used to determine many things about the light source. e.i. the composition of elements or gases of the source, among other uses. How scientists determine results is by measuring the amplitude and intensity of each color in the spectrum.
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Use a Bradford assay for protein. This uses a dye which goes from red to blue when it binds to proteins, and the colour change and hence the amount of protein present cana be assessed spectrophotometrically. This is a cheap, sensitive method and will give you a quantitative answer (providing you calibrate your spectrophotometer first using a standard curve produced with solutions of different known protein concentrations). This link will tell you more about the exact method.\n\nhttp://www-class.unl.edu/biochem/protein_assay/bradford_assay.htm
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Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance is watts per steradian per square metre (W·sr-1·m-2).
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Some molecules absorb visible light, e.g. due to conjugation and pi to pi antibonding transitions, with the highest intensity at a particular wavelength.  One can prepare standard solutions, with accurately devised known concentrations, the measured intensities of these solutions are correlated with these standard solutions, covering a particular dynamic range.  Assuming Beer's Law, the plot will resemble a linear curve, and a least fit regression line and its equation can be obtained; also by correcting for the blank if this is has not bee accounted for by the spectrometer.  The unknown pure solution, or a matrix derived alternative method if the solution is not pure that there are significant impurities in the unknown solution, will also be measured at this wavelength and it's intensity, often the y value, will be plugged into the regression equation to estimate the concentration.
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instrumental chemistry is the branch of chemistry where various instruments (spectroscopy: NMR, NearIR, XRay, UV/Visible, fluorescence, atomic emission,...; electrochemistry, etc.) are used to study the behavior of chemical and molecular systems.  The radiation source depends on the technique and can be as simple as a lightbulb (albeit a special one) in UV/Vis or NIR spectroscopy to an XRay source (like the one at the doctor's office) in XRay spectroscopy.  The different techniques have different applications, depending on what you want to study.
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Litmus is a water-soluble dye extracted from certain lichens and absorbed on to filter paper. The resulting piece of paper becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions, the color change occurring over the pH range 4.5-8.3 (at 25°C). Neutral litmus paper is purple in colour. The active ingredient of Litmus is called Erythrolitmin, and commercial litmus dye is chemically p-Dimethylaminoazobenzenesulfonic acid, sodium salt.
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The visible light region is from 0.7 µm to 0.4µm of the spectrum. The color of an object that we see is the color or wavelength of light reflected from the visible spectrum. All other colors or wavelength are absorbed. the following chemicals are different in terms of wavelength because of their different nature's regarding reflection of particular wavelength or the absorption of other.
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it's reactive to light
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different types of chlorophyls are composed of different pigments which absorb different wavelengths of light (different colors in the spectrum)
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A spectrum analyzer is a device or system that decomposes a wavetrain or waveform into the individual frequencies that make up the wave.  Usually, one displays the power in a series of frequency (or wavelength) intervals that cover the relevant range of frequencies.  The input waveform can be any time-varying signal, e.g., a sound wave, RF wave, electromagnetic wave, etc.\n\nThe various "bars and waves" visualizations in Windows Media Player are actually just a spectrum analyzer display of the music being played. Low frequency components are on the left and higher frequencies are on the right. Unfortunately, the display isn't calibrated, and we're not told either what frequency range is covered (probably something like 20-20,000 Hz), or the frequency ranges of each interval.  Nevertheless, it's fun to watch!
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Volumetric analysis is limited by precision of calibration and may be affected by concerns such as temperature and mixing.\n\n"Spectrometry" may cover a wide variety of technologies, but if we consider absorption spectroscopy techniques limitations include Beer's law deviations, noise, calibration error, and interfering components in the case of impure samples.\n\nColorimetry can be thought of as absorption spectroscopy taken at a single wavelength.  The limitations are similar, but the affects of interfering components will be harder to detect.
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The UV-VIS spectrum is due to the electronic transitions of the molecule. This is characteristic of a compound. Qualitative and quantitative estimations of compounds are possible by this non destructive technique. \n\nThe absorption (A) of a solution at a particular wavelength is given by Beer-Lambert's law ect = A where c is the concentration of the compound , t is the thickness of the cell and e is the molar extinction coefficient characteristic of the compound at a given wavelength. This principle is used for quantitative measurements. \n\nCary 5E is a high resolution spectrophotometer to study the electronic spectrum of solutions, single crystals and powder samples (in nujol mull) in the range of 3150-185 nm. This is a double beam instrument controlled by a microprocessor. Base line correction, repetitive scan, kinetics, derivative and concentration modes are possible. Diffuse reflectance is available for solid samples. VT accessory in the range 5-90 degrees Centigrade is available\n\nThis principle is used for quantitative measurements.
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A radiometer does what you describe. \n\nhttp://store.sundancesolar.com/solrad.html\n\n\nDirections to build a simple radiometer:\n\nMake a radiometer to measure solar energy. Colour the paper side of a gum wrapper black and cut in four pieces. Stick the pieces to each side of a match stick with the shiny sides all facing the same way. Wrap a thread around a pencil and attach to the match stick. Suspend in the jar and place in a sunny location. The radiometer will turn as it absorbs energy.
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The Beer-Lambert Law forms the basis of Spectroscopy, an analytical tool for measuring concentrations.  Spectrophotometers are instruments used in laboratories to measure concentrations.  The most common is a UV/Vis spectrophotometer.  This instrument passes light(in this case UV or visible light) through a fixed length of solution and measures the radiation absorbed at a particular wavelentgh.  Thus the instrument gives an absobance reading of the solution.  Since the thickness or length of the solution is constant (the solution is placed in a glass cuvette, a little glass square that is of known length/thickness) from the Beer-Lambert law, the absorption is proportional to the concentration.  So, you prepare say three solutions of chlorophyll that are of three known concentrations (a high, middle and low standard) and measure their absorbance.  Now you measure the absorbance of your unknown sample.  Finally you use the mathematics of linear regression, to extrapolate the concentration of your unknown sample.\n\nThis is the most common and easiest way to measure concentrations in an analytical laboratory.
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Brix is the measure of % sugar in a given sample. The instrument which is used to measure brix is called a refractometer.