Filters

Question 1

Role of filters

Answer 1

They take away part of the light. Important for use in clinical evaluation. Optical filters are able to remove some wavelengths selectively.

Question 2

How do color filters work?

Answer 2

They absorb and/or reflect some incident wavelengths while transmitting others.
The filter SUBTRACTS light to produce color. Less lumens exit the filter than entered.

Question 3

Transmission curves

1. Long pass filters
2. Short pass filters
3. Band pass filters

Answer 3

1. Transmits all wavelengths greater than specified. 500 long pass filter? Will transmit 500 nm and above.
2. Transmits all wavelengths shorter than specified. 600 short pass filter? Will transmit 600nm and below.
3. Transmits a specific band of wavelengths. EX: 550/20 band pass filter. Transmits 20nm, 10 on each side of 550.
   So 540-560nm.

Question 4

What does a heat absorbing filter do?

Answer 4

Glass designed to minimize infrared transmission and maximize visible range transmission.

Can get warm because it is absorbing most of the IR light.
Can add coatings to lenses to keep even more IR out.

Question 5

Percent transmission trend of crown glass, CR39, and poly

Answer 5

Crown glass and CR39 have the same transmission, but crown glass lets more UV light through at the same time (more to the left on the graph, where numbers are smaller). Crown glass typically needs a coating to counter this.

CR39 transmits more light than polycarbonate and some people will notice this.

Graph: Crown glass to the left, CR39 in the middle and poly on the right.
Crown glass and CR39 are the same height, and poly is shorter in height.

X- wavelength
Y- Percent transmission.

Question 6

What happens when you combine filters?

Answer 6

Each filter will block their designated wavelength, with the remaining wavelengths passing through.

Each filter subtracts their wavelength.

Question 7

What are neutral density filters?

Answer 7

Filters that transmit equally across the visible spectrum. The darker the filter, the less light transmission.
Ex: Sunglasses

Question 8

Optical density (OD)

Answer 8

Used to specify the degree of attenuation (loss of light) of the filter.
OD are additive.

OD= log(1/T)

Clinically, very important. Can help quantify an APD, or differentiate between types of amblyopia (developmental or pathological). Ex: How much filter is needed over the good eye to make the bad eye turn on?

Question 9

Interference filters

Answer 9

Very important to optometric equipment, and not as necessary for patient interaction.

The filters transmit a selected wavelength region with very high efficiency. Used when you want a very narrow wavelength to pass through.
Ex: Fluorescein angiography

Question 10

Dichroic filters

Answer 10

Important to equipment. Can be long pass or short pass. Part of the light is reflected through filter 1 at 90 degrees and part of the light is transmitted through filter 2, which is straight ahead.

Great to use when we want the light coming off the patients retina to go to two different detectors.