A pulse oximeter is a convenient non intrusive measuring device. Usually, it has a pair of small light-emitting diodes facing a photodiode through a translucent part of the patient's body, which is typically a fingertip or an earlove. One LED is read, with a wavelength of 660 nanometers, while the other is 905, 910 or 940-nanometer infrared. Absorption at these wavelengths varies considerably between oxyhemoglobin and its deoxygenated form. Thus, the oxyhemoglobin and deoxyhemoglobin ratio can be measured from the ratio of the absorption of the red and infrared light. The absorbtion of oxyhemoglobin and deoxyhemoglobin is practically the same, called the isosbestic point. Earlier oximeters used the wavelengths of 590 and 805 nanometers for correction of the concentration of hemoglobin.