Principles of aberrometers
May 31st
Principles of aberrometers:
Aberrometers are usually classified into three types. The first type is the outgoing wavefront aberrometer as in the Hartmann–Shack sensor, and the second type is the ingoing retinal imaging aberrometer as in the cross-cylinder aberrometer, Tscherning aberrometer and the sequential retinal ray tracing method. The third type is the ingoing feedback aberrometer as used in the spatially resolved refractometer11 and the optical path difference method.
why my child feel blur this week?
May 30th
why my child feel blur this week?
I always met boys or girls under 14 years old who complained that I could not see clearly recent weeks. Parents took them to optometry doctor to get new pair of glasses. After you put on positive lens on them, and a while, you will find no more glasses were need. The boys or girl were always watching TV or computer for a long time or reading for a highly long tense. At this circumstances, you have to prescribe mydriatic to them instead of prescribing new eye glasses. A review should be performed two weeks later.
Can you swim after lasik?
May 25th
Swimming after lasik?
Swimming after lasik is allowed, but if you are in six months after surgery, no swimming is recommended. Because the water containing the disinfectant in the pool would destroy the recovery strength between the corneal layers.
How much does lasik eye surgery cost?
May 25th
How much does lasik eye surgery cost?
You first go to hospital for a thorough exam for your eyes. If you are wearing your soft contact lens, put off them for at least 2 weeks; if you are wearing your rigid gas permeable contact lens, just put off them for at least one month before you go to your doctor.
After routine examinations such as visual acuity, subjective and objective, intraocular pressure, corneal topography, diameter of the pupil, corneal thickness, length of axis and sometimes wavefront aberrations of your eye, doctor will analyse the data and tell you if you are suitable for refractive surgery. After analyzing the data, the surgery plan will be made as a function of your corneal thickness. Lasik is the most popular surgery plan in current. But now there are evidences to show surface-ablation-like is in regression. The surgery cost of lasik is about $1000. PRK is a little less. But the medications after PRK are more and long.
What would cause blindness from bleeding behind the eye?
May 25th
Trauma is the first reason of the blindness from bleeding behind the eye. Such as orbital compound fractures and orbital emphysema(often seen in medial orbital wall fractures) that are caused by fighting, skiting, falling and traffic accidents related to the head and faces.
The second reason is hypertension or high level blood-fat and cholesterin that would occlude the vessels behind the eye. Occlusion would finally lead to bleeding.
Tumor, general blood system diseases like leucocythemia are the uncommon etiological factor.
Schwartz syndrome: retinal detachment with high intraocular pressure
May 11th
Schwartz syndrome: retinal detachment with high intraocular pressure
A famous painter of 40 years old man in local came to me with his left eye seeing blur and covered for a month. No swelling feelings of his both eyes. But the intraocular pressure are OD 55mmHg, OS 35mmHg. I dilated both eyes. His left retina were nearly fully detached. Right eye had 4 hole in different three peripheral zones. So scleral buckling was performed on the right eye and vitrectomy performed on the left eye. A lot of tiny holes were found in the left eye during operation process. Pneumatic retinopexy was inevitablely.
Retinal detachment always accompanies with low intraocular pressure. But Schwartz syndrome do the opposite. The reason is that aqueous humor secretion increases when peripheral retina drawed by the contraction of the vitreous body. And inflammations occurs at the trabecular meshwork. When the retinal detachment is repaired, the IOP will down.
Principles of four wavefront aberrometers
May 9th
The Tscherning aberroscope projects a grid of points onto the retina, the distortion of the grid of points is photographed, and the wavefront aberrations are calculated from this distortion.
The Hartmann-Shack wavefront sensor technique relies on the projection of a point source light on the retina. The light reflected from this point on the retina is imaged onto a lenslet array and sampled with a charged coupled device chip. Wavefront aberrations are calculated assuming that a point of light on the retina was the origin of the image.
The Skiaskopy technique effectively performs retinoscopy at multiple points over the pupil, and from these data, multiple refractive errors of the wavefront over the entire exit pupil were calculated.
The laser ray-tracing technique, which we used in this study, projects a thin infrared laser beam onto the retina and detects the location of reflection one by one in succession. This is sequentially completed over 64 sampling points within 0.06 seconds, so that the eye movement does not affect the measurement. The merit of this technique is that it can detect larger magnitude errors of reflection than Hartmann-Shack methods and that it is more rapid and sensitive than the Tscherning methods that essentially require averaging of data from each sampling point.
Principles of high-resolution Shack-Hartmann Sensor Aberrometry
A low-power, invisible, plane laser-wave is directed into the eye.
It is focused on the retina by the refractive power of the cornea and the lens.
The retina absorbs part of this light, the other part is reflected and goes back through the optical media of the eye.
The optical aberrations along the light path within the eye lead to distortions of the wavefront exiting the eye.
The distorted wavefront hits the Shack-Hartmann Sensor consisting of an array of 1,500 micro-lenses and a low-light CCD sensor in the focal plane of the lenses.
The deviations of all spots from their regular positions (in reference to an incident wave) are measured and from these values the optical aberrations are computed in the form of Zernike polynomials.
Advantages:
High resolution
High accuracy and reproducibility
Fast measuring & analysis
Wide dynamic range
High resolution data need powerful software and hardware tools for evaluation of aberrations
Disadvantages:
Wavefront may not be suitable for all patients. In order to determine if you are a good candidate for the wavefront treatment, come in for a pre-operative evaluation at our Clinic.
Principles of Tscherning Aberrometry:
A collimated laser beam illuminates a mask with a regular matrix of 168 pinholes to form a bundle of thin parallel rays
These rays form a spot pattern on the retina that is more or less distorted according to the optical errors of the eye
This retinal spot pattern is imaged onto the sensor of a low light CCD camera by indirect ophthalmoscopy
The deviations of all spots from the form of Zernike polynomials
Advantages:
Fast measuring
Disadvantages:
Time-consuming set-up
Time-consuming analysis
Low resolution
Diffraction of the laser radiation at the small pin holes disturbs the result
Complicated mechanical and optical arrangement
Patient disturbed by visible flash (green, 532 nm)