IR cut filter and infrared photography

 the proper name is NIR cutoff or blocking filter

 

first optical LPF(LPF1) is anti-aliasing filter ( to reduce Moire effect) ( can be 2 glasses) and where dust removal piezo is attached to

IR cut filter or ICF( sometimes called LPF2) is the filter to be replaced for IR conversion

types of IR cut filters: transmissive (left) and reflective (right). The transmissive type is also commonly called blue glass.

The top 2 photo is taken directly over the filters, and bottom photo taken slightly slanted

common ICF are BG38 ( QB21) and BG39 (QB39)

Schott BG18 spec S8612 

BG40  and some others

in order to do infrared photography, it is needed to replace this IR cut filter with a IR band pass filter:

Eg GG420(JB420) 420nm to >2500nm or BCF700 400 to 710 nm

note the difference of 

Ha increase

nebulosity

bright stars blur

colour doped filter (old) vs sputtered interfering filter:

 

The transition from blocking of light to transmission of light is lazy with colored glass. Transmission (T) and Optical Density (blocking level, known as OD) plots reveal a very shallow transition slope. For example, RG630, a common red glass from Schott glass, requires approx. 70nm of spectral wavelength range to go from OD4 to 90%T! That’s a long transition, reducing the precision of the desired contrast. This same filter also requires 40-45nm of spectral range to go from 2%T to 90%T. 

Interference filters consist of thin films of material coated onto various clear glasses.  An optical coating engineer can build a particular coating using an almost endless number of different combinations of alternating layers of materials with low and high indices of refraction. 

Typical materials used in sputtered interference coatings include oxides of metals with very high melting points and silicon dioxide. Metallic salts (zinc sulfide, yttrium chloride etc.) are used along with silicon dioxide in thermal resistive interference coating methods (“soft-coated” filters). Coatings are often referred to as dielectric coatings, but sputtered and electron-beam coatings can also include conductive metal layers (aluminum, silver, etc.).

from Jeff Carmichael of Chroma

Olympus

interference filter calculator

IR mod for astrophotography

lifepixel on how to diy infrared conversion. 

BCF(BCF700) Baader conversion filter for canon DSLR

kolarivision

cctvopticallens

Hayata 

 

wolfcrow 

astrosurf

迪卡

cirphoto forum 

cirphoto ICF and IR filters 

kleptography

Hiroyuki Yoneshima (米島宏幸)

modification service

夢天天文, TB

tan14