Chameleon slightly puffs throat when siting under basking light?

When I see my chameleon under the basking light I see him puffing SLIGHTLY but idk if it's stress or if he is just trying to absorb more heat like that??? I have a dimmer on the basking lamp so I don't put it high at all I always pit it low... Btw he always does that when he's under the light.
 
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