Why do spectrophotometers rarely measure absorbance values greater than 2?

Answer 1

The definition of absorbance is A=log( I(0) / I), which is the logarithm (base 10) of the initial intensity of the light passed through the sample divided by the intensity of the light after it has passed through. Let's look at a few values of A and see what these values imply. If A = 0, then I(0) must equal I, which means the intensity doesn't change, which means no light is absorbed. If A = 1, then I(0) / I must equal 10. This means that 90% of the light was absorbed. If A=2, then I(0) / I must equal 100, and 99% of the light was absorbed. If A=3, then I(0) / I must equal 1000, and 99.9% of the light was absorbed. And so on. To measure A over a range of 0 to 2 means that the spectrophotometer must be able to measure light intensities as small as 1/100th of the max value. This is a range of 100x, which is about what you can expect with ordinary electronic equipment. Compare this range to the ranges on a voltmeter. My voltmeter has several ranges available. Each of them has a range of only 10x. Instead of building multiple ranges into your spectrophotometer, the manufacturer (to save you money) expects you to dilute your sample until it falls into the usable range of the device.