Hi, Mou...

First, the image must not be in RGB format.. you can choose 8 bits (it will be shown in gray level, don't worry). Now you can go to IMAGE > LOOKUP TABLES and choose the RED one (so you get back to the original colors). Now you can set a calibration in the Analyze > Calibrate menu.   You can enter your calibration (left column = X; right column = Y) and choose a Function. Then, you can go to ANALYZE > TOOLS > CALIBRATION BARS.. So you can have the calibration embedded in the image. 

Hope I could help you... 

All the best

Thank you so much. But I am still confusing about this issue. You mentioned the left column x and the right column y, what does that mean? And also, the function what should I choose? If there's a calibration bar, could I determine the intensity when I choose a cell? Thank you Dr. Nogueira. 

Dear Mou. The columns you use to set a calibration, for example: if what is black in the image represents 0 mg of calcium (just an e.g.), and the strongest red is 25 mg of calcium. In this case, you can make a calibration (in this case using only 2 points, the zero and 25... ). So, as black is represented in ImageJ (8 bits images) by zero, and white (red in the case you choose it in the lookup table) is 255.. so you can make

in the left column:

0

255

in the right column:

0

25

and choose the straight line for the function. 

After calibrated, you can select an ROI (region of interest) by choosing the forms (rectangle, circle, etc.) available, and then go to Analyze>measure (CTRL+M) and you will have a statistics of the region you selected.  You can also go to SET Measurements to select what do you want to measure...

Hope it is what you are looking for...

All the best