absorbance value at 475 nm
Wavelength (nm)

https://answers.yahoo.com/question/index?qid=20101030062418A...
Why do we "blank" the spectrophotometer inbetween changing cuvettes?

http://www.explorebiology.com/documents/Lab17Spectrophotomet...
With the blank cuvette in, rotate the light control knob (right-hand knob) until the meter scale reads 100% transmittance. Remove the blank, close the cuvette holder top. The meter should now read 0% transmittance.
If the machine now reads 0% transmittance with the blank out and 100% transmittance with the blank in — the spectrophotometer has been “zeroed in.”

Absorbance
Absorbance is a logarithmic scale with unequal divisions. A value of 0.0 absorbance means that
no light has been absorbed by the material and all the light has been transmitted. A reading all
the way at the other end of the scale means that all the light has been absorbed by the material.
WHAT IS THE “BLANK” AND WHY DO WE HAVE TO USE IT?
All substances, clear or not, absorb some light. Therefore, if we wish to measure the
absorbance (or the transmittance) of a material like protein in solution, we must not only
consider the light absorbed by the protein, but also the light absorbed by the cuvette and the
light absorbed by the solution in which the protein is dissolved.
In order to measure only the absorbance of the protein in the solution, we must “subtract” out
the absorbance due to the cuvette and the solvent in which the protein is dissolved.
If our protein is dissolved in water, and we are trying to assess how much protein is in solution,
we would ultimately want to measure how much light is absorbed by the protein alone and not
the how much light is absorbed by either the water or the cuvette. In order to subtract out these
factors, we prepare another cuvette with only water in it. This is called the “blank.” In general,
the blank is a cuvette which contains everything that is in the sample (or experimental) cuvette,
except the one material whose absorbance we are measuring.
To use the blank, you could measure your experimental cuvette, then measure your blank
cuvette and find the difference between them. However the spectrophotometer has a built in
calibration system that allows you to avoid this calculation and its much like zeroing an
electronic scale. Follow this procedure:
1. Before you measure a solution in an experimental cuvette, first insert the blank cuvette into
the machine. The spectrophotometer will register an absorbance value for the blank.
2. Now adjust the lower right hand dial to manually set the meter back to zero absorbance.
3. Take the blank out and now you are ready to read the absorbance of the material in solution
without having your data affected by the absorbance of the cuvette or the solvent.
4. Also be aware that fingerprints on the cuvettes can affect your readings, so all cuvettes
need to be wiped clean with a Kimwipe before inserting them into the spectrophotometer.

虽然从翻译角度来说这样翻译没有问题，但是如果按照这个翻译去操作基本会让人无所适从，不知道怎么做。当然这不是翻译的错，是原文叙述的问题。

分光光度计是无所谓调零的，也没法调零。只需要用空白对照管进行纠正后就可以了，这时候你如果读空白管会是零。这里所谓的空白对照管就是A管（不加入酪氨酸酶）。