optometer

Question 1

outline simple optometer (3)

Answer 1

• optical device used for measuring refractive state of eye
• subjective
• single optometer lens + movable target

Question 2

how is ocular refraction determined (1)

difference between myope and hyperope measuring (2)

Answer 2

• determined by measuring far point of the eye (Mr, k)
• myopic subjects has far point at finite distance in front of retina -> just need to place object closer until it appears clear
• hyperopes has far point behind retina -> converging rays can be produced from real objects by placing a postive lens in front of eye

Question 3

what is special about badal optometer (2)

Answer 3

• contains a target and positive power lens (badal lens) like a simple optometer
• but secondary focal point F’ of optom lens coincides at eye principal point P or at spectacle point S

Question 4

badal optometer advantages (3)

Answer 4

• linear dioptric power scale
• constant angular image size regardless of target location
• movement closer to eye doesn’t stimulate accom

Question 5

simple optometer disadvantages (4)

Answer 5

• non-linear dioptric (power) scale
• involuntary accomodation (subject knows where object is) -> difficult with young subjects (accom)
• results are often slightly myopic
• no astigmatism