Meaning of "optical" in "Optical cap, 8X Strip" (used in qPCR) I'm translating an English document that lists equipment used in a qPCR procedure: > **Reagents/Materials** > > * Optical cap, 8X Strip > I googled and found that the meaning of this line is "a strip of 8 caps for PCR tubes". What is the meaning of **optical** though? I'm not sure how to translate it into Russian. Does the word indicate the the cap is fully transparent? Or that the cap creates no visual distortion (it is flat)? Screenshot: > ![enter image description here]( I'm googling but I can't seem to find any definition of "optical cap". It is used in a lot of books and documents as if the term is self-evident, while it's not.

> Does the word indicate the the cap is fully transparent? Or that the cap creates no visual distortion (it is flat)?

Yes and yes. As an example of the different kinds of PCR tube caps, refer to the cap options available from Thomas Scientific:

> Available with a domed, flat frosted, or **flat clear caps for qPCR**. [emphasis mine]

For most lab plastics, "optical" refers to the surface between the camera/detector and the sample. Another example: for plate-based fluorescence assays where the plate reader takes measurements from below the plate, one might use a Nunc MicroWell 96-Well _Optical-Bottom_ Plate from ThermoFisher. Usually, such plates also have opaque walls to prevent signal bleed from adjacent wells.