Westheimer and Liang 
proposed to analyze the intensity distribution of the DP images to extract information about the combined contribution of the ocular aberrations and the intraocular scattering on the retinal image. In a later work, they combined the information of the DP images with subjective measurements to separate out the influence of the intraocular scattering on the visual performance 
. However, they did not evaluate the contribution of the ocular aberrations on the DP images. In addition, in those studies the procedure was not applied in eyes where scatter was the main degrading effect.
We have further refined this approach to evaluate the contribution of scatter separated from refractive errors and higher order aberrations, with the main intention to classify the degree of cataracts. The main consequence of the definition of the scale of values of OSI is that we can establish a comparison between the particular positions of any of the analyzed eyes in this classification and the position of the same eye in the gradation made by the LOCSIII system. This analysis would give a percentage of agreement regarding to the number of eyes classified in equivalent levels by the two scales.
It should be noted the significant advantages of the combined use of both approaches. OSI gradation directly relates with the visual degradation (forward scatter), while the information in the slit-lamp images is useful to understand the nature and type of the cataract, although it is not always simple to predict the actual visual impact. The DP image is affected by both the forward and the backward scattering, produced in the first and the second pass of the light through the crystalline lens; the analysis of the energy distribution on this image reveals the contribution of the light scattering which really impairs visual performance.
We found an agreement of 75% by considering the complete population between the two schemes (OSI versus LOCSIII). This means that 40 of the 53 eyes were sorted as equivalent by using both methods. Although this percentage is relevant, in the daily clinical practice the capability of detecting an early cataract is especially useful and therefore, it would be very interesting to show the agreement between the eyes ranked in one of the two first levels in both scales. The percentage of agreement between the eyes classified in one of the two first levels in both scales was 84%. This means that 21 of the 25 eyes, which were ranked by the values of OSI on the first and second levels, were classified accordingly to the LOCSIII chart. This further emphasizes that a value of OSI lower than 1 characterizes a non-cataract eye, whereas a value of OSI around 2 would correspond with an early cataract eye, corresponding to NO2 in the LOCSIII chart. With regard to the eyes classified in the third and fourth level of the OSI scale, they were all mature cataracts. In this case, the disagreement between LOCSIII and the OSI rankings was not quite relevant since, from a practical point of view, it does not affect the diagnosis capability of early cataract process.
The correspondence between the classification made by clinicians, by the LOCSIII chart, and the gradation established from the values of OSI, supports the applicability of this objective method to make a sound decision on the proper timing for a cataract surgery. Moreover the quantification provided by an objective gradation as OSI, can be also very useful to complete the available data from anamnesis, or reduced visual acuity in the diagnosis of an early cataract process.
In addition, it would be important to evaluate the values of OSI in other types of lens opacification, as well as to refine the possible ranges for classifying these cataracts depending on the intraocular scattering measured by OQAS. Having new data about intraocular scattering in cataracts may also be helpful to understand the developing process on cataracts, as well as to better understand the way the crystalline lens evolves from being transparent to increased dispersion as well as other morphological and optical changes, as could be the increased aberrations with age 
Patients mainly affected by nuclear cataract were primarily selected among a large number of potential candidates by using slit-lamp images. However, some concomitancy between nuclear with cortical or capsular cataracts is not possible to avoid. Different types of cataracts may have a different contribution on the ocular scattering. The impossibility for estimating whether there was or not some contribution of other type of cataract together with the predominant nuclear type, could be considered as one of the factors to explain the differences between the gradation made by the LOCS and the objective classification based on OSI.
The DP image, and therefore the scatter index, is affected by the complete eye. It would be possible in some particular cases to measure an elevated OSI due to corneal haze, which is not related to cataract. In the DP instrument, the laser beam in the first pass has a small diameter (1.5 mm diameter) and reaches the eye within the pupil center area. With a localized cataract the actual impact location of the beam could affect directly the image in the retina. In the second pass, after retinal reflection, the situation is different since the light fills the whole area of the pupil, and hence, the entire crystalline lens (within the area of the pupil) is considered. DP image actually contains information from the whole pupil area. However, some minor variability in the results could be observed due to the impact of the first pass. In an instrument with a large beam also in the first pass, this problem could be avoided. We should emphasize that the reported method provides average information within the complete pupil area; i.e, it is not spatially resolved. This technique, although useful, should be combined with complementary techniques routinely. The DP images were recorded using near infrared light. This is an advantage for the patient's comfort during image acquisition. However, the magnitude of scatter in infrared light can be different than in visible light. The impact of retinal scatter could be also larger in infrared since light penetrates deeper in the retina. This is a limitation for the absolute characterization of scatter, but not very relevant for the relative gradation of cataract of this study.
The changes in the cornea and the vitreous as a function of age may also increase the overall amount of ocular scattering. However, in non pathological cases, the magnitude of these effects would be smaller than that of the lens in early cataract.
In conclusion, a new objective optical method has been developed to quantify the degree of cataract. It is based on the recording and processing of double pass retinal images of a point source. We demonstrated the potential of the technique in a group of cataract patients. The correlation of the cataract gradation between this approach and a standard subjective method (LOCSIII) is significant.
However the higher sensitivity and the intrinsic nature of this approach to detect forward scattering renders our method more powerful specially in the detection of earlier stages of cataract and to relate cataract symptoms with visual complaints. The method is robust and fully based on objective measurements; i.e., not depending on subjective decisions. This procedure could be used in combination with standard current methods to improve cataract surgery patients scheduling.