**Purpose.**:
To determine the dependency of straylight on ocular biometry.

**Methods.**:
This prospective study included 518 eyes of 277 volunteers of diverse ethnic backgrounds with healthy eyes of various iris colors. The eyes had retinal straylight tested using a commercial psychophysical device. Ocular axial length and refraction were also measured with an ocular biometer and an autorefractometer, respectively.

**Results.**:
The measured retinal straylight was validated by comparing data with the age model described in the literature as log(*s*) = *P* _{1} + log[1 + (age/65)^{4}], where *P* _{1} is the logarithm of the average straylight for the eyes of a newborn. The data agreed well with this model, although *P* _{1} was slightly higher (0.931 vs. 0.87). When this model was subtracted from the measured straylight values, a quadratic increase was found in the function of axial length, *L*: log(*s*) = 0.931 + log[1 + (age/65)^{4}] + (0.01089*L* ^{2} − 0.4820*L* + 5.330). A similar model was defined for the spherical equivalent refraction *SE*. This corresponds to an increasing amount of straylight for increasing degrees of myopia. No correlation was found with keratometry and corneal astigmatism or with iris color.

**Conclusions.**:
Retinal straylight increases not only with age, but also with axial length. Further study is needed to identify the cause of this dependency.

^{ 1,2 }

^{ 3,4 }corneal edema,

^{ 5 }Fuchs' corneal dystrophy, and vitreous floaters are known to increase retinal straylight considerably, which may lead to symptoms such as loss of contrast sensitivity, disability glare, and halos. These phenomena reduce a patient's quality of vision in everyday life—for example, while driving at night and in recognizing a person against the background of a light source—but have only a very limited effect on visual acuity, as measured during an ophthalmic examination.

^{ 3 }

^{ 6,7 }Furthermore, the literature reports higher values in eyes with light iris colors than in those with dark iris colors because of fundus reflectance and the translucency of the iris and eye wall.

^{ 8 }Especially in albino patients, light iris color causes considerably increased straylight, partially due to backscattered light originating from the choroid that is not absorbed because of the absence of retinal pigment.

*s*) can be modeled as follows

^{ 4 }: where

*P*

_{1}= 0.87 is the logarithm of the average straylight in a newborn's eyes (base);

*P*

_{2}= 65 is the age that retinal straylight,

*s*, doubles; and

*P*

_{3}= 4 is the power. This increase to the fourth power of age was found to be valid for a range of scattering angles.

^{ 6 }

*n*= 189), as well as from a group of patients who were pre-LASEK (

*n*= 88). Any subjects with a history of ocular surgery, amblyopia, early cataract, corneal haze, corneal scars, or systemic diseases (e.g., diabetes, systemic macula diseases) were excluded, as well as pregnant women and hard contact lens wearers. As Project Gullstrand aims to describe the general population, no additional selection criteria were used.

^{ 9,10 }(C-Quant; Oculus Optikgeräte, Wetzlar, Germany). This method has been described in full detail in the literature

^{ 9,10 }and has been thoroughly validated.

^{ 11,12 }It provides a measure for the straylight parameter log(

*s*), as well as an estimation of the fit quality

*Q*of the psychometric function and a repeated-measures estimated SD (

*Esd*).

^{ 13 }In the following, only measurements with an

*Esd*parameter below 0.08 and a measurement quality parameter

*Q*> 0.5 were included. Each measurement was performed under spherical equivalent correction of the patient's refraction by means of added lenses.

^{ 7 }, a numerical value was assigned to each eye color to account for the iris.

*Esd*parameter below 0.08 and a measurement quality parameter

*Q*above 0.5) was obtained. Axial length, anterior biometry, and autorefractometer measurements were also available for 518 of these eyes and have been included in this study. The population data of these subjects are given in Table 1.

*s*) is shown as a function of age in Figure 1. Straylight remained constant until the age of 45, after which it gradually increased. On average, the data appeared to be higher than the values given by model 1 (solid line in Fig. 1), particularly in the age range of 20 and 40 years. If these eyes are divided as a function of their spherical equivalent (SE) refraction, the young adult eyes with high straylight correspond with higher myopia values (data series with SE < −3 D) and with higher hyperopia to a lesser degree (series with SE > +1 D). It must be noted that because of the inclusion of prerefractive patients in the study population, there was an overrepresentation of young, medium myopic eyes.

*P*

_{1},

*P*

_{2}, and

*P*

_{3}of model 1 can be estimated by means of a least-squares fit. For this purpose, only the 189 emmetropic eyes in this group (i.e., eyes with SE refractions between −1 and +1 D) were considered, eliminating any possible influence of the SE refraction.

*P*

_{1},

*P*

_{2}, and

*P*

_{3}of model 1 to these data gives a coefficient of determination of

*r*

^{2}= 0.331. If instead we choose

*P*

_{3}= 4, as is proposed in the literature, and fit

*P*

_{1}and

*P*

_{2}to the data, we find

*r*

^{2}= 0.321. Finally, if only parameter

*P*

_{1}is fitted and the values

*P*

_{2}= 65 and

*P*

_{3}= 4 proposed in the literature are used,

*r*

^{2}= 0.319 is found (Table 2). As there did not seem to be large differences in coefficient of determination between these three models and to avoid overfitting of the data, we decided to use the single-parameter model in the following, with

*P*

_{1}= 0.931.

*r*

^{2}= 0.105, while fitting it to a parabola results in

*r*

^{2}= 0.137, with

*P*< 0.0005 for the quadratic component, which justifies the use of a quadratic fit.

*s*is found to increase as a function of the axial length

*L*. Fitting a parabola to these data produces a coefficient of determination of

*r*

^{2}= 0.159 (

*P*< 0.0005 for the quadratic component; Fig. 2b), whereas a linear fit results in

*r*

^{2}= 0.122. Combining this parabolic relationship with the age model 1 gives the following equation: leading to the coefficient of determination is

*r*

^{2}= 0.285. A comparison of the

*r*

^{2}values for models 1, 2, and 3 is given in Table 3, both for the entire population and for the subpopulation of 191 emmetropic eyes. Note that the coefficient of determination

*r*

^{2}for the age model is much smaller for the entire group than when only emmetropic subjects were included, because of the large number of ametropic individuals included in our population.

Model | All Eyes | Emmetropic Eyes | |
---|---|---|---|

Age model | (1) | 0.038 | 0.319 |

Age and SE model | (2) | 0.265 | 0.308 |

Age and L model | (3) | 0.285 | 0.328 |

Eyes, n | 518 | 189 |

*r*

^{2}= 0.010), or with astigmatism (

*r*

^{2}= 0.003). No significant difference was found when the BAC straylight of 334 eyes with with-the-rule astigmatism larger than −0.5 D was compared to 33 eyes with against-the-rule astigmatism larger than −0.5 D (unpaired

*t*-test,

*P*= 0.470). A correlation was found between

*SE*and

*L*(

*r*

^{2}= 0.643).

*n*= 156), green (

*n*= 66), brown (

*n*= 111), and black (

*n*= 27). Most of the black eyes belonged to non-Caucasian subjects. To conform to the literature,

^{ 7 }numerical values were assigned to each eye color to account for iris pigmentation (blue:

*C*= 1.2; green:

*C*= 1.0; brown:

*C*= 0.5; black:

*C*= 0.0). These values can be used to include iris color in the form of a linear term added to model 3.

*P*

_{1}, the linear and quadratic coefficients of the SE refraction, and the linear coefficient of eye color

*C*gives the following: which leads to a coefficient of determination of

*r*

^{2}= 0.331, almost the same as for model 3, in which the eye color term was not included (

*r*

^{2}= 0.315), and much higher than the age model 1 (

*r*

^{2}= 0.171).

*P*= 0.837). The SD taken over the entire group with respect to the SE model 2 is 0.14 log units.

^{ 4 }are suitable to describe the age-related increase in retinal straylight in this data set as well. The only exception is

*P*

_{1}, for which the slightly higher value of 0.931 was found to be more appropriate. Speculatively, this may be the result of small calibration differences or differences in spectral distribution of the light used between the compensation comparison device used in the literature

^{ 4 }and the one that we used (C-Quant; Oculus), or differences between the populations studied. In the earlier study, active drivers were recruited, which may have caused a slight bias.

*SE*is found that becomes apparent only after the age model 1 is subtracted from the measured straylight data (Fig. 2a). This result suggests that including linear and quadratic terms of the axial length into the straylight model may improve it and provide a better fit of the measurements obtained in this study. This possibility is confirmed by the coefficient of determination

*r*

^{2}of model 2, which is substantially higher than that of age model 1 (Table 3). The use of the spherical equivalent is justified, since corneal astigmatism has been shown not to have any influence on BAC straylight (

*r*

^{2}= 0.003).

*L*plays a major role in ocular refraction (

*r*

^{2}= 0.643), it is not surprising that a similar relationship was found between BAC straylight and

*L*(Fig. 2b).

*SE*data from a previous population study

^{ 4 }were analyzed further by one of the authors (TvdB) in a way similar to the present study (model 2). These data also showed a significant quadratic increase with SE, albeit by less than half of that found in the present study.

^{ 4 }an average of two repeated measures was used to calibrate the C-Quant, whereas in the present study only one measurement per eye was used. Moreover, in the latter study only subjects with clear lenses were selected as a reference for the C-Quant device, whereas the Gullstrand study did not impose such a criterion.

^{ 7 }iris pigmentation did not appear to have a noticeable influence on the straylight measurements in this data set. This discrepancy may partly be due to the higher SD in the present data compared with that in the literature, resulting in an insufficient statistical power to detect a significant effect.

*I*, which increases as a function of axial length

*L*. Smaller image sizes would produce smaller test patterns on the retina, which results in a test angle that is smaller than the 7° test angle of the C-Quant. A 15% decrease in image size would result in a test angle of 6°, which corresponds with a straylight increase of 0.01 log units (calculated using the wide angle straylight models for the standard observer published by the CIE

^{ 14 }). Similarly smaller straylight decreases are found for increases in image size. As the range of axial lengths in our populations corresponds with a range of image sizes between ±15%, this effect causes straylight changes of −0.007 to +0.01 log units. However this is an order of magnitude too small to explain the observations in Figure 2.

*SE*is unclear at this point and, despite the obvious improvement in statistical fit and coefficient of determination with respect to model 1, only a fraction of the variation in retinal straylight is explained by these new models.

*Invest Ophthalmol Vis Sci*. 1997;38:1321–1332. [PubMed]

*Vision Res*. 1999;39:1437–1445. [CrossRef] [PubMed]

*Invest Ophthalmol Vis Sci*. 1992;33:618–625. [PubMed]

*Am J Ophthalmol*. 2007;144:358–363. [CrossRef] [PubMed]

*Ophthalmic Physiol Opt*. 2002;22:209–213. [CrossRef] [PubMed]

*Vision Res*. 1990;30:699–707. [CrossRef] [PubMed]

*Optom Vis Sci*. 1995;72:52–59. [CrossRef] [PubMed]

*Vision Res*. 1991;31:1361–1367. [CrossRef] [PubMed]

*Invest Ophthalmol Vis Sci*. 2006;47:768–776. [CrossRef] [PubMed]

*J Vision*. 2007:29;7:8. [CrossRef]

*J Biomed Opt*. 2006;11:054010.

*Br J Ophthalmol*. 2008;92:788–791. [CrossRef] [PubMed]

*J Biomed Opt*. 2006;11:34027. [CrossRef] [PubMed]

*CIE Collection*. 1999;135(1):1–9.