AXIAL LENGTH: Axial length is the distance between anterior and posterior pole of eyeball
 At birth 17 mm
 In adult 24 mm
 In myopia more than 24 mm
 In hyperopia less than 24 mm
METHODS: There are two methods
A. Ultrasonic
B. Optical
ULTRASONIC METHOD:
i. It contain a 10MHz solid probe with inbuilt fixation
ii. It has to be aligned with optical axis of the eye for accurate measurement
iii. This can be done in two technique :
A. Immersion technique
B. Contact technique
1. IMMERSION TECHNIQUE:
--------Patient lie in supine position looking a fixation at the ceiling
--------A scleral shell is applied to the eye filled with 1% OR 2% Methylcellulose
--------Probe is Immersed in the solution keeping 5-10 mm away from the cornea
--------Probe is gently moved intil properly aligned with optical axis
--------Echo spike will displayed in the monitor.

ECHO SPIKES ARE:-
--- IS = INITIAL SPIKE PRODUCED BY PROBE
--- C1 = ANTERIOR CORNEAL SPIKE
--- C2 = POSTERIOR CORNEAL SPIKE
--- L1 = ANTERIOR LENS SPIKE
--- L2 = POSTERIOR LENS SPIKE
--- R = RETINAL SPIKE
--- S = SCLERAL SPIKE
--- O = ORBITAL SPIKE
2. CONTACT TECHNIQUE:
i. Alternative to immersion technique doesn’t use scleral shell
ii. Probe directly contact with cornea
iii. These can be done in two methods:
1. Hand held
2. Attaching with slit lamp or applanation tonometer

PROCEDURE:
------ Patients seated after applying local anesthetic drop
------ Asked to fixed a target straight with non-testing eye
------ Probe is properly aligned along with visual axis & brought forward to touch the cornea
------ Five spike will be displayed on the screen.

ECHO SPIKES ARE:
--- C = POSTERIOR COrNEAL SPIKE
--- A = ANTERIOR LENS SPIKE
--- P = POSTERIOR LENS SPIKE
--- R = RETINAL SPIKE
--- S = SCLERAL SPIKE
B.. OPTICAL TECHNIQUE:
i. Use partial coherence laser
ii. Biometer measure the time required for infrared light to travel to retina
iii. This technique doesn’t have contact with cornea, so corneal compression
artifacts are eliminated.

FOR DETAILS NOTE;
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Binocular Subjective Refraction:
Part-1: [ Ссылка ]
Part-2: [ Ссылка ]
Part -3: [ Ссылка ]
Part -4: [ Ссылка ]
Part-5: [ Ссылка ]

Biometry:
Part-1: [ Ссылка ]
Paet-2: [ Ссылка ]
Part-3 [ Ссылка ]

Optical Coherence Topography:
Full: [ Ссылка ]
Part-1: [ Ссылка ]
Part-2: [ Ссылка ]

Dry Eye:
Part-1: [ Ссылка ]
Part -2: [ Ссылка ]
Part -3: [ Ссылка ]
Part -4: [ Ссылка ]
Part -5: [ Ссылка ]
Part- 6: [ Ссылка ]
Full: [ Ссылка ]

Myopia
Part-1: [ Ссылка ]
Part-2: [ Ссылка ]
Part-3: [ Ссылка ]
Part -4: [ Ссылка ]
Part -5: [ Ссылка ]

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