TO Caps & Lenses

A Transistor Outline (TO) is the name of an international industrial standard that governs a particular type of current-conducting electronic housing. A TO package always consists of two components: a header and a cap. In opto-electronics, the caps fulfill two primary functions. First, they reliably and permanently protect the optical components of transmission as well as reception applications. Second, they ensure the transmission of the optical signals by acting as an optical interface. The optical properties of the windows or lenses installed into the TO caps must therefore fulfill extremely high requirements.

In general, one can distinguish between two different types of TO caps:

Molded TO Caps & Lenses

Metal and glass are fused without the use of any other material. These TO caps are suitable for all conventional types of optical signal transmission. Read more >

Soldered TO Caps & Lenses

Metal and glass are fused with the aid of a solder glass. These TO cap are suitable for precision applications with special optical components. Read more >


The headers and caps are welded together to form a TO housing. During this process, the oxygen inside the encapsulated spaces is removed to increase protection against corrosion. Please refer to our Technology section for more information about this process.

Both types of caps can be manufactured for all standard TO designs. The following table lists some of the most common designs and their associated header designs:
cap
type
ID mm
OD mm
FD mm
compatible to
TO 5 L, W, FW, UFW 7.7 8.1 9.1 TO 5, TO 39
TO 8 W 11.8 12.3 14 TO 8
TO 18   4.2 4.6 5.3 TO 18, TO 46
TO 38   2.5 2.7 3.4 TO 38
TO 39          
TO 41 TW        
TO 46          
TO 52 L, W, FW, TW, UFW 4.3 4.7 5.4 TO 18, TO 46
TO 56 W, TW, UFW 3.3 3.5 4.2 TO 41, TO 56
Machined   custom      
Pigtail   1.7 1.9 2.4 Pigtail
Pillpack   custom      
Rectangular   custom      

Molded Caps: L = Lens, W = Window

Solder Caps: BL = Ball Lens, FW = Filter Window, TW = Tilted Window, WW = Wedge Window, UFW = Ultra Flat Window
For more information on "High Refractive Index Ball Lens Caps", please refer to the Solder Lens Cap page or the download section.
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