Last week we found that oplev light quantity is reducing.

So at first we thought that this issue come from power supply of light source of the AC 100V.

So I checked this voltage, however we was not able to find any issue.
I confirmed the light brightness of the QPD side visually, and it was very weak.(Fig.1)
Then I confirmed the light brightness of the light source side visually too, and maybe I think it was not so bad.(Fig.2)
Therefore I thought that this issue come from light path, so I confirmed the view port.
As this result, we found the frost on the view port of the 4K shield.(Fig.3 : TM oplev QPD side)
We stopped the all 4K cryocooler for warming up.
After turning off these cryocooler, this frost is gradually vanishing.(Fig.4 : TM oplev light source side, Fig.5 : TM oplev QPD side)
Around ~16:50, we confirmed the light brightness became light.(Fig.6 : TM oplev QPD side)
Tomorrow We will check this view port again.