500 series instruments (as well as other spectral instruments) occasionally require a full calibration, which includes a black calibration. The proper procedure for performing a full calibration follows:
Begin by scrolling to Configuration > Cal Options > Full Calibration. You will be propted to read your white calibration tile twice.
Then locate the triangle shaped plastic piece on the back of your 500, it is located below the power and communication ports. You will find that you can rotate this piece, which allows the instrument shoe to open until it is perpendicular to the body of the instrument.
You will now want to hold the instrument in such a fashion that when we take a reading no light is reflected back to the optics. Most people find that holding the instrument under a desk in a darkened room works well. Be sure to hold the instrument at an angle so the light from the lamps does not reflect directly back towards the instrument.
Use a finger to depress the read switch and continue holding it down while the instrument takes a series of four readings. The read switch is the small plunger located in front of the optics assembly on the bottom of the 500. Be sure that nothing is in front of the optics when you do this (watch out for your other fingers), and do not release the switch until the four readings are complete. The black calibration is now complete. You can close the shoe and rotate the triangle plastic stop back to its original position.