Filter transmission vs optical density? The same thing or completely different?

[pipnina]

Hi! My chroma narrowband filters just arrived and they gave me two charts with each one. One chart is as seen on the shopping page (% transmission, in a range of 300 to 1100nm, so these 3nm filters are very hard to see the true useful bandpass)

But they also came with a second graph of "optical density" which was not on the FLO website (I presume since each batch may be slightly different?). This graph's scan range is of only 646 to 666nm on the Ha filter, so the true bandpass is easy to see, assuming what i'm looking at is the same as the first graph, but inverted? It seems to show that my filter would pass 100% from 655.5 to 657.5, and only be slightly tapered at 658. This might be good since I was initially worried that even some closer/larger galaxies like M51, M82 and M66 would be down in the depths of the filter's downward curve after I did some redshift maths, but this might mean I have more sensitivity to the red end than I expected?

[wimvb]

The OD is not a linear scale rather

10^-OD

This means that if OD = 0, all light is passed

If OD = 1, 10% of the light is passed, 90% is blocked

OD = 2, 1% is passed, 99% is blocked

OD = 3, 0.1% is passed, 99.9% is blocked, etc.

OD is a measure of how much light outside the pass band is blocked. A good (high OD) nb filter will block much of the light outside the transmission band, and yield better contrast than a poor one (low OD)

Edited May 19, 2022 by wimvb

[pipnina]

11 hours ago, wimvb said:

The OD is not a linear scale rather

10^-OD

This means that if OD = 0, all light is passed

If OD = 1, 10% of the light is passed, 90% is blocked

OD = 2, 1% is passed, 99% is blocked

OD = 3, 0.1% is passed, 99.9% is blocked, etc.

OD is a measure of how much light outside the pass band is blocked. A good (high OD) nb filter will block much of the light outside the transmission band, and yield better contrast than a poor one (low OD)

Ah I see!

Many thanks

[wimvb]

I believe it was @ollypenrice who once said that filters are not about what they pass through, but rather what they block. A nb filter with higher OD will be better at blocking light pollution, and give you more contrast. Filters with high OD are more difficult to make, and are more expensive.