Today we start a series of posts devoted to providing information, tips, tricks and answers to frequently asked questions by Calitoo users!

So, how do you measure Aerosol Optical Thickness (AOT) with a Calitoo?

First and foremost: NEVER look directly at the sun!

And…ONLY take measurements when the sun is not obstructed by clouds.

TIP: Use your hand(s) to cover the sun; if you see any clouds on either side of your hand(s), do NOT take any measurements. Wait until the sun is not obstructed by clouds.

Now, the Calitoo sun photometer measures AOT values at 465 nm, 540 nm and 619 nm.  Are you ready to take some measurements? Follow the next steps:

  1. Turn on the instrument by pressing and holding down the button for two seconds.
  2. Wait until a GPS signal is found for your location. Screen will show “>> or <<” when searching and “3D” when GPS is locked on:

NOTE: Once your instrument is on, holding down the button again for two seconds will switch between Measuring Mode and Reading Mode. In order to take your measurements, make sure you are on Measuring Mode!

The Measuring Mode displays:

Real time Red, Blue and Green wavelength data

UTC time

Barometric pressure


GPS location





3. Point the Calitoo photometer at the sun. Remember, do NOT look directly at the sun! Find the sun dot. Align the sun dot with the center of the target.







4. Press the button once. The raw signal numbers on the screen will fluctuate.

5. Keeping the photometer steady with the sun dot centered on target, press the button again when the raw signal is maximized.







6. This screen displays the AOT values you just collected.









OK, you took your AOT measurements so you should write them down on your journal. But also, you can record them on your Calitoo! Here’s how:


Press the button again. The next screen will say “Recording”, the number specific to the data point you just collected, and “Ok?”



To store the data, press and hold the button, until the bottom of the screen says “Recorded!”

When you let go of the button, the photometer will go back to measuring mode for more data collection. 

TIP: Holding down the button too long while trying to store data will NOT store the data, only switch from the Measuring Mode to Reading Mode.


TRICK: Also, if you don’t hold down the button long enough while trying to store data, the data will NOT be stored and you will go back to the first screen. Only hold down the button to store data until the screen says “Recorded!” Then the data will be stored on the photometer.


Once you have recorded your first set of AOT measurements, you’re ready to start all over! Make a total of three measurement trials each time you go out.


For more information, go to:

To watch a video about taking measurements with a Calitoo, go to:

Stay tuned and read our next Calitoo Corner post!



More Blog Entries