**Working principle**

Light from a LED light source passes through a cuvette containing a solution sample, as shown in the following picture. Some of the incoming light is absorbed by the solution. As a result, the light of a lower intensity strikes a photodiode.

**Transmittance and Absorbance**

The amount of light that passes through a solution is known as transmittance. Transmittance can be expressed as the ratio of the intensity of the transmitted light. IT_{2} and the initial intensity of the light beam, Io_{2},as expressed by the formula：

T=I_{T }/ I_{O}

The Colorimeter produces an output voltage which varies in a linear way with transmittance, allowing a computer, calculator, or handheld to monitor transmittance data for a solution. The reciprocal of transmittance of the sample varies logarithmically (base ten) with the product of three factors: ε the molar absorptivity of the solution, , the cell or cuvette width, and C, the molar concentration.

log (1/T) = ε LC

In addition, many experiments designed to use a handheld color meter require a related measurement, absorbance. At first glance, the relationship between transmittance and absorbance would appear to be a simple inverse relationship; that is, as the amount of light transmitted by a solution increases, the amount of light absorbed might be expected to decrease proportionally. But the true relationship between these two variables is inverse and logarithmic (base 10). It can be expressed as：

A=log (1/T)

Combining the two previous equations, the following expression is obtained: A=ε LC

In effect, this formula implies that the light absorbed by a solution depends on the absorbing ability of the solute, the distance traveled by the light through the solution, and the concentration of the solution. For a given solution contained in a cuvette with constant cell width, one can assume and b to be constant. This leads to the equation：

A=k · C

where k is a proportionality constant. This equation shows absorbance to be related directly to concentration and represents a mathematical statement of Beer’s law. Beer’s law is discussed in more detail below. In this guide and in some of our computer programs, the transmittance is expressed as percent transmittance or %T. Since T=%T/100, the formula can be rewritten as：

A=log (100/%T) or A=2-log%T

**Using the Colorimeter**

The Colorimeter is easy to use and maintain. Simply connect it to your data collection interface, configure your software, and you are ready to make measurements. For best results, let the system stabilize at the desired wavelength for 5 minutes prior to calibration or data collection.

**Warranty**

In addition to this，ATO colorimeters warrant this product to be free from defects in materials and workmanship for a period of five years from the date of shipment to the customer. This warranty does not cover damage to the product caused by abuse or improper use.