Working Distance Calculations in Retinoscopy

Static Retinoscopy can be done in two ways; using a working distance (WD) lens in front of the patient’s eye or without it.

A working distance lens is used to compensate for the distance between the examiner and the patient’s eye. Usually, the working distance is taken as arm length i.e 66cm (+1.50DS)

Dioptric distances(working distance) Compare the width of spaces A and B. Each is 8 cm wide, yet there is a tenfold difference in dioptric width.

  • When a working distance lens is used to neutralize the reflex, the total lens system in front of the patient’s eyes is considered as Gross retinoscopic (GR) value and if the WD lens value is deducted from it, the remaining power is considered as Net retinoscopic (NR) value; which is taken as the final value of retinoscopy.
  • Formula GR value = WD lens value + NR value




Examples:

  • If a Working Distance lens of +1.50 DS is used and the reflex is neutralized with -3.00DS then,

NR= -3.00DS; WD= +1.50 DS

GR= (+1.50) + (-3.00) = -1.5DS

So, the power of the lens with which reflex is neutralized (irrespective of the power of WD lens) i.e NR value is taken as the final retinoscopy value. Here, -3.00DS is taken as the final power of retinoscopy.

  • If WD is +2.00DS and reflex is neutralized with -3.00DS/-1.00DC × 180 then

NR=-3.00DS/ -1.00DC × 180

GR = (+2.00) + (-3.00 DS / -1.00DC × 180)

= +1.00DS/ -1.00DC × 180

The final retinoscopy power is taken as -3.00DS/-1.00DC × 180

Note: Working Distance lens value is deduced only from the spherical power of NR value. In the above example, +2.00DS is reduced from -3.00DS.

  • When retinoscopy is performed without using a WD lens the total power in front of the patient’s eye is equal to GR value.

(GR value) – (the distance from retinoscope head to the patient’s eye) gives the NR value.

Example:

  • If the reflex is neutralized with -3.50 DS and the working distance between retinoscope head and patient’s eye is +1.50DS then

GR= -3.50DS; WD= +1.50DS

NR= (- 3.50) – (+1.50)

= -5.00DS.

The value of final retinoscope power is taken as -5.00DS

  • If the reflex is neutralized with +1.00DS and WD is taken as +2.00 DS then

GR= +1.00DS

NR= (+1.00) – (+2.00)

= -1.00DS

The final retinoscopy power is -1.00DS

Note: Always subtract the Working distance value from the neutralized lens power to get the final value; while WD lens is not used.

  • If the reflex is neutralized with -1.00DS/-1.00DC×180 and WD is +2.00 then

GR = -1.00DS/-1.00DC×180

NR= (-1.00DS/-1.00DC×180) – (+2.00DS)

= -3.00DS/-1.00DC×180 {cylinder remains unchanged}

The final power is taken as -3.00DS/-1.00DC×180

Sushma November 10, 2016 Clinical Procedures