as cataract and refractive surgery are increasingly performed , the surgeons ' skills as well as the precision of ocular measurements are important in order to satisfy patients ' expectations . central corneal thickness ( cct ) is critical in designing vision correction surgeries such as laser in situ keratomileusis ( lasik ) , as well as in glaucoma diagnosis and other corneal diseases [ 16 ] . measurements of the anterior chamber depth ( acd ) , which is defined as the distance from the corneal epithelium to the anterior surface of the crystalline lens , and the aqueous depth ( ad ) , which is defined as the distance from the corneal endothelium to the anterior surface of crystalline lens , have many clinical applications . the acd measurement is used , for example , by the holladay 2 formula , whereas the ad measurement is critical for the selection of patients undergoing phakic intraocular lens implantation . anterior segment optical coherence tomography ( visante as - oct , carl zeiss meditec , dublin , ca , usa ) has been clinically used for several years for anterior segment measurement and has high resolution . it is based on low coherence interferometry and uses the light source of a 1310  nm superluminescent light - emitting diode . the visante as - oct is widely used to measure the corneal thickness and acd . the al - scan is a newly introduced optical biometer that can measure six parameters within 10 seconds , including cct , acd , axial length ( al ) , corneal keratometry ( k ) , white - to - white ( wtw ) , and pupil diameter ( pd ) . it uses the principle of the scheimpflug imaging to measure cct and acd and an 830  nm infrared laser diode for al . previous studies had reported highly repeatable and reproducible measurements of al , k values , and acd with this new device [ 914 ] . few studies have investigated the accuracy and agreement of al - scan with other instruments [ 9 , 10 , 1315 ] . this is the first study to compare the ocular measurements obtained by the al - scan and the visante as - oct . sixty - three eyes of 63 healthy subjects ( 36 men , 27 women ) were enrolled in the study . mean age ( standard deviation , sd ) was 23  3.83 years ( range : 1832 years ) . mean refraction error was 4.41  2.12  d ( range : 0.5  d to 9.00  d ) . the exclusion criteria were age < 18 years , previous ocular surgery , anterior or posterior pathology , contact lens usage ( within 4 weeks for rigid contact lens and within 2 weeks for soft contact lens ) , and astigmatism > 3.0  d. before enrolment , each patient underwent a complete ophthalmological examination , including visual acuity , intraocular pressure measurement , anterior segment evaluation , and fundus examination . this study was approved by the review board of the eye hospital of wenzhou medical university and performed according to the declaration of helsinki . al - scan uses the principle of the scheimpflug imaging to measure cct and acd with 470  nm monochromatic light emitted from an led . the anterior chamber single - scan mode was used to measure the cct and acd with visante oct . the depth and width of the scanning field were 6.0  mm and 16.0  mm , respectively . the scan was obtained when a vertical white line along the center of the cornea was visible . the calibrated caliper function was used to calculate the acd and ad [ 16 , 17 ] . all measurements were taken between 10:00 and 17:00 and were completed within 15 minutes for each patient . spss software version 21.0 ( ibm corporation , armonk , ny , usa ) and medcalc statistical software v14.8.1 ( medcalc software , inc . , the kolmogorov - smirnov test was performed to check the data distribution for each device . the paired t - test was used to evaluate the difference between the measurements of each device . the range of agreement was shown with 95% limits of agreement ( loa ) , which stands for the mean difference  1.96  sd . the mean cct , acd , and ad measured by al - scan were 538.59  27.37  m , 3.70  0.30  mm , and 3.16  0.30  mm , respectively . the visante oct showed 536.14  26.61  m for cct , 3.71  0.29  mm for acd , and 3.17  0.29  mm for ad . although there was a statistically significant difference in the mean cct measurements between the two devices , it was clinically insignificant ( table 1 ) . good agreement was found between the two devices for cct with a maximum boundary value of 95% loa of 14.35  m ( figure 1 ) . the acd and ad measurements of al - scan and visante oct were similar ( p > 0.05 ) and had good agreement with the 95% loa range of 0.15 to 0.12  mm ( figures 2 and 3 ) . accurate quantitative measurements of cct , acd , and ad provide valuable clinical information and are important for preoperative assessment , surgical planning , and follow - up in phakic iol implantation . ultrasound ( us ) is typically widely used for measuring these parameters [ 19 , 20 ] . but , nowadays , noncontact devices such as the visante as - oct are more popular in measuring ocular parameters . the al - scan is a recently released , noncontact , imaging instrument using partial coherence interferometry ( pci ) and the scheimpflug principle with good repeatability and reproducibility . the scheimpflug camera with a 470  nm led is used for measuring the cct and anterior chamber in the al - scan . our data is the first study to suggest that the visante as - oct and the al - scan have good agreement for measuring cct , acd , and ad . previous studies have investigated the al - scan and compared it to other instruments , mainly the iolmaster . since the iolmaster is unable to directly measure the corneal thickness , no prior data for comparing cct were available . ethnic variation was found in previous studies when cct values were measured ; chinese , caucasians , hispanics , and filipinos had comparable cct measurements , whereas japanese had significantly thinner corneas than caucasians , chinese , filipinos , and hispanics . the differences also existed between the anterior chamber of chinese and caucasians [ 2123 ] . so we are more focused on the repeatability results rather than the mean measurement values . . showed high repeatability of cct values by the al - scan in both normal and keratoconic groups . although its reproducibility was not better than other available scheimpflug systems , the al - scan showed excellent and comparable repeatability and reproducibility in most ocular parameters ' measurements [ 1012 , 24 , 25 ] . thus , it was useful to review and compare the currently used devices such as pentacam ( oculus , wetzlar , germany ) , galilei ( ziemer , port , switzerland ) , and sirius ( costruzione strumenti oftalmici , florence , italy ) as they all use the principle of the scheimpflug imaging to measure the cct despite the lack of direct comparison between the al - scan and other scheimpflug systems . a recent analysis showed that the total measurement error of visante oct for cct was 7.88  m , while the error was 9.85  m , 7.05  m , 2.64  m , and 4.76  m for ultrasound , pentacam , galilei , and sirius , respectively . .   showed low coefficients of repeatability and reproducibility and high intraclass correlation coefficients of the cct measurement by visante oct . in our recent prospective studies on three different scheimpflug imaging systems and one oct , high repeatability and good agreement for cct measurement were also demonstrated [ 28 , 31 ] . however , o'donnell et al .   showed that the 95% loa for pentacam and visante oct were 25.61 to 49.11  m . in the current study , the visante oct provided slightly thinner cct than al - scan , which was also seen between visante and pentacam by nemeth et al . , and our max boundary of 95% loa was 14.35  m , demonstrating very good agreement between the two devices . the scheimpflug and oct measure cct by different optical and physical techniques : the pentacam used 475  nm blue light , visante oct used 1310  nm diode laser , and al - scan used 470  nm led , which might contribute to the differences in the results . besides , the anterior corneal surface also influences the demarcating boundary , which results in differences . a previous study had shown that the total measurement error of iolmaster for acd was 0.06  mm and the error of visante oct , pentacam , and galilei was all approximately 0.05  mm . as compared to the iolmaster 500 , the acd was 3.17  0.12  mm by al - scan and 3.12  0.11  mm by iolmaster , with a minor difference of 0.13  0.04  mm and high correlation between al - scan and iolmaster 500 in measuring acd . srivannaboon et al .   also showed a small difference indicating good agreement between al - scan and iolmaster with a loa range of 0.24 to 0.19  mm , which was similar to the results of our previous comparison between the al - scan and iolmaster . nemeth et al . , wang et al . , and lavanya et al .   showed that the acd measurements by oct were 3.11  0.33  mm , 3.76  0.21  mm , and 3.14  0.34  mm , respectively , in normal adults and presented good agreement with ultrasound or scheimpflug or iolmaster . .   demonstrated that acd measured by visante oct had deeper but not clinically important values than iolmaster . reported high correlation between pentacam and visante oct in measuring acd , which was similar in keratoconus in a study by yazici et al . . in our current study , the 95% loa range was even narrower for acd or ad measurements indicating better agreement between al - scan and visante oct than that between al - scan and iolmaster or between al - scan and galilei . in our current study we simultaneously measured acd and ad modes , which is more comprehensive than other studies that only analyzed one mode . in clinical settings , acd and the intraocular pressure are important parameters for glaucoma screening and diagnosis . however , the acd values are the summation of cct and ad values . since acd values can be affected by cct measurement , the method used for measuring cct , the accuracy of cct measurement , corneal edema , and other aspects related to cct results will influence the precision of acd . thus , it was meaningful to assess the agreement of these parameters between the two devices in a single study . we only included healthy unoperated eyes and further investigations are needed to assess both instruments for other categories of patients ( such as those affected with keratoconus or previous refractive surgery ) . mydriasis would influence changes in the cornea and anterior chamber , so further studies will be performed to evaluate the performance of the biometer after pupil dilation . this study found a clinically insignificant difference between the two devices for the measurement of cct . the al - scan and visante as - oct have good agreement in measuring cct , acd , and ad , and their results can be interchangeably used in the clinical setting .
purpose . to compare between a new optical biometer ( al - scan , nidek co. , aichi , japan ) and an anterior segment optical coherence tomographer ( visante as - oct , carl zeiss meditec , dublin , usa ) for measuring central corneal thickness ( cct ) , anterior chamber depth ( acd ) , and aqueous depth ( ad ) 
 . methods . 
 sixty - three eyes of 63 normal subjects were examined with al - scan and visante as - oct in this prospective study . 
 one eye per subject was measured three times with both devices to record their cct , acd , and ad . 
 all procedures were performed by the same operator . 
 agreement between the two devices was assessed using paired t - tests , bland - altman plots , and 95% limits of agreement ( loa ) . 
 results . 
 the mean cct , acd , and ad measured by al - scan were 538.59  27.37  m , 3.70  0.30  mm , and 3.16  0.30  mm , respectively . 
 the mean values obtained by the visante oct were 536.14  26.61  m for cct , 3.71  0.29  mm for acd , and 3.17  0.29  mm for ad . 
 the mean cct by the al - scan was higher than that obtained by the visante as - oct ( difference = 2.45  6.07  m , p < 0.05 ) . 
 the differences in acd and ad measurements were not statistically significant . 
 the 95% loa of cct , acd , and ad were between 9.44 and 14.35  m , 0.15 and 0.12  mm , and 0.15 and 0.12  mm , respectively . 
 conclusions . since these two devices were comparable for measuring cct , acd , and ad , their results can be interchangeably used in the clinic .