====== Calibrate... ======

**Use this dialog box to calibrate an image to a set of density standards.**

Use this dialog box to calibrate an image to a set of density standards, for example radioactive isotope standards or a calibrated optical density step tablet.

Before using this command, use **Analyze>Clear Results** to reset the measurement counter and use one of the selection tools and **Analyze>Measure** to record the mean gray value of each of the standards. There is an [[http://rsb.info.nih.gov/ij/docs/examples/calibration|example]] that shows how to calibrate to an optical density step tablet.

[[|{{ http://rsb.info.nih.gov/ij/docs/images/calibrate.gif?303x350 | calibration table dialog}}]]

When finished making the measurements, select **Analyze>Calibrate** to display the **Calibrate** dialog box. To calibrate the image, enter the known standard values in the right column, select a curve fitting method from the drop down menu, enter the unit of measurement, and click OK. ImageJ will then display the calibration function. 

[[|{{ http://rsb.info.nih.gov/ij/docs/images/calibration-function.gif?547x332 |calibration function display}}]]

If the calibration function is not satisfactory, bring up the Calibrate dialog box again and select a different curve fitting method.

**Rodbard** is a four parameter general curve fit function proposed by David Rodbard at NIH. The form of the equation is: \\
<latex>
   y = d + \frac{(a - d)} {(1 + (\frac{x}{c})_b )}
</latex>

Selecting **Uncalibrated OD** from the drop down menu causes ImageJ to convert gray values to uncalibrated optical density values using the function:

<latex>
  Uncalibrated OD = log_{10} \frac{255}{PixelValue}
</latex>

You do not need to measure OD standards or enter known OD values to enable this feature.