**Haag-Streit LenStar LS900**

The LenStar LS900 has been the first competitor of the IOLMaster and the first optical biometer to measure lens thickness (LT). Its technology is based on optical low-coherence reflectometry, with partial coherence superposition of light waves emitted from an 820 nm super luminescent diode to measure the axial length (AL) of the eye. The instrument also measures central corneal thickness (CCT) and aqueous depth (AQD), defined as the measurement from the corneal endothelium to the anterior lens surface. The latter 2 are added to provide the anterior chamber depth (ACD, from corneal epithelium to anterior lens surface). In addition, the instrument measures retinal thickness and corneal diameter (CD). The keratometry (K) readings are calculated by analyzing the anterior corneal curvature at 32 reference points orientated in 2 circles at approximately the 2.30 mm and 1.65 mm optical zones. Alternatively, a T-cone topographer (with 11 Placido rings of the central 6.0 mm) is available. The size and centricity of the pupil are also measured.

The LenStar provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1, Olsen and SRK/T formulas; moreover, it is the only biometer including the Radial Basis Function (RBF) method to calculate the IOL power. Toric calculation is performed with Barrett’s calculator. For post-LASIK eyes, the Barrett True-K, Masket and Shammas No-History formulas are available.

**Heidelberg Anterion**

The Anterion** **is a SS-OCT with a 1300 nm wavelength light source, with a speed of 50000 A-scans/second. This technology is used to measure AL and LT, as well as CCT and aqueous depth (AQD), which is defined as the distance from the central posterior corneal surface to the central anterior lens surface. In order to measure ACD, CCT and AQD have to be added.

The Anterion images the anterior segment of the eye with an axial depth of 14 mm, a lateral width of 16.5 mm, an in tissue axial resolution of <10 µm and lateral resolution of 30-45 mm. All measurements are assisted by an eye-tracking technology, centered on the corneal vertex. Corneal information is based on 65 radial scans, generating data for corneal maps and analysis at 8 mm diameter. Using the Cataract App the following parameters can be analyzed:

· Simulated keratometry (Sim-K) average: this is defined as the average corneal power calculated at a 3 mm ring, based on anterior corneal axial curvature. The conversion of anterior radii to keratometry values is performed using the standard keratometric index of 1.3375.

· Total corneal power (TCP) average: this is defined as the average refractive power of the cornea, calculated by ray tracing at a 3 mm ring through both corneal surfaces. Ray tracing determines how parallel light beams are refracted according to the slope of the cornea, the true refractive indices (ncornea = 1.376, naqueous = 1.336), and the exact point of refraction.

The Anterion provides IOL power calculation by the Barrett Universal II, Haigis, Hoffer Q, Holladay 1 and SRK/T formulas. Moreover, ray-tracing can be performed by Okulix. For post-LASIK eyes, the Barrett True-K and Haigis-L formulas are available.

**Movu/Alcon Argos**

The Argos is an optical biometer using a 1060 nm wavelength and 20 nm bandwidth SS-OCT technology to collect 2-dimensional OCT data of the whole eye. The device provides 3 OCT images in every acquisition to measure the AL, ACD, CCT, AQD, LT, pupil size, and CD. It is the only optical biometer calculating the AL without relying on the group refractive index originally developed for the IOLMaster, but by means of segmented AL, where the optical distances are converted into geometric distances using the standard refractive indices of 1.376 for the cornea, 1.336 for the aqueous and vitreous, and 1.410 for the lens. Keratometry is obtained from OCT information in combination with a ring LED.

The Argos provides IOL power calculation by the Barrett Universal II, Haigis, Hoffer Q, Holladay 1 and SRK/T formulas. For toric IOLs, the Barrett calculator is available. For post-LASIK eyes, the Barrett True-K and Shammas No-History formulas are available.

By entering postoperative refraction, it is possible to optimize formula constants in retrospect.

**Nidek AL-Scan**

The AL-Scan** **is based on PCI. It measures the axial length (AL) of the eye with partial coherence superposition of light waves emitted from an 830 nm super luminescent diode. It uses a 970 nm light-emitting diode (LED) for K and pupil diameter and a 525 nm LED for determination of CD. Corneal curvature is measured by analyzing the images of double mires of spots (360°) at diameters of 2.4 mm and 3.3 mm reflected from the anterior surface of the cornea. The corneal power is calculated by using the keratometric index of n = 1.3375. In order to use the optimized constants of the ULIB constants, the 2.4-mm diameter is usually selected. In addition, the unit uses a Scheimpflug camera with a 470 nm light-emitting diode monochromatic light to measure the CCT and the ACD.

The AL-Scan provides IOL power calculation by the Barrett II Universal, Camellin-Calossi, Haigis, Hoffer Q, Holladay 1 and SRK/T formulas, as well as toric calculation with Barrett’s calculator. For post-LASIK eyes, the Barrett True-K and Camellin-Calossi formulas are available.

In cases where AL cannot be measured because of an extremely dense cataract, the AL-Scan is the only optical biometer that provides an optional built-in ultrasound biometer.

**Oculus Pentacam AXL Wave**

The Pentacam AXL Wave is the latest development of the well-known rotating Scheimpflug camera. Whereas the standard Pentacam AXL is equipped with an AL measurement module that relies on PCI with a blue-light emitting diode (wavelength of 475 nm) as the light source, the new Pentacam AXL Wave incorporates also a Hartmann-Shack aberrometer. Therefore, this is the only biometer that can differentiate among aberrations from the anterior corneal surface, posterior corneal surface and the internal diopters.

Corneal curvature is measured by a rotating Scheimpflug camera that generates 25 meridional scans. From these data several corneal power values are calculated:

Standard K: this is calculated by converting the measured radius of the corneal surface into diopters using the standard 1.3375 keratometric refractive index.

Total Corneal Refractive Power (TCRP): this is calculated by ray-tracing through both corneal surfaces using the corneal index of refraction (n = 1.376). It can be centered on the pupil or the corneal vertex and can be calculated along a ring (with a diameter from 2 to 8 mm) or inside it (so-called zone).

True Net Power (TNP): this is calculated by a modified Gaussian optic formulla through both corneal surfaces using the corneal index of refraction (n = 1.376). It can also be centered on the pupil or the corneal vertex and can be calculated along a ring (with a diameter from 2 to 8 mm) or inside it (so-called zone).

Both TCRP and TNP provide corneal power values that are lower with respect to standard K, so that they cannot be entered into standard formulas unless specific constant optimization is performed.

The presence of the Scheimpflug camera provide users also with the many maps and measurements of the standard Pentacam, such as the Belin-Ambrosio display for keratoconus as well as corneal and lens densitometry.

The Pentacam AXL Wave provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1, Olsen and SRK/T formulas, as well as toric calculation with Barrett’s and Savini’s calculator (the latter is based on total corneal astigmatism). For post-LASIK eyes, the Barrett True-K, Hill-Potvin-Shammas and Olsen formulas are available.

**Optopol Revo NX**

The Revo NX is an anterior-posterior segment spectral domain OCT; a superluminescent diode laser (wavelength 830 nm) is the light source in this device. It has an axial resolution of 5 μm, transverse resolution of 12 μm and scan depth of 2.4 mm and obtains 110 000 scans per second. A superluminescent laser diode (830 nm) is used as the light source. The optical biometry program within the device measures the AL, ACD, LT and CCT.

The corneal topography module provides users with anterior, posterior and total corneal power maps.

IOL power calculation is still under development.

**Tomey OA-2000**

The OA-2000** **combines an optical biometer, based on SS-OCT, and a Placido ring topographer. It can measure AL, K, ACD, LT, CD, CCT and pupil diameter. The SS-OCT uses a wavelength of 1060 nm. The Placido disk corneal topographer, which projects 9 mire rings, can simultaneously measure the radius of curvature of the cornea at diameters of 2.0, 2.5 mm and 3.0 mm; K is calculated using the keratometric index of n = 1.3375 and the 2.5 mm is usually selected in order to rely on optimized constants from the ULIB website. Corneal maps with a diameter of 5.5 mm can be displayed.

The anterior segment of the eye is illuminated by infrared light and the image captured by a charge-coupled device camera: the captured image is analysed to calculate the pupil diameter and CD.

The OA-2000 provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1, Olsen and SRK/T formulas, as well as toric calculation with Barrett’s calculator. For post-LASIK eyes, the Shammas No-History and the Double-K SRK/T formulas are available.

By entering postoperative refraction, it is possible to optimize formula constants in retrospect.

**Topcon Aladdin**

The Aladdin combines an optical biometer and a Placido-ring topographer. Optical biometry is based on optical low-coherence interferometry, with an 830 nm superluminescent diode that is used to measure the AL, ACD, CCT and LT of the eye. Optical distances are converted into geometric distances using a group refractive index. Corneal topography (with axial and tangential maps) is based on the reflection of 24 Placido disk rings with a diameter of 8.0 mm. Corneal power values are derived from automated K, whose values are generated from the reflection of 4 dedicated Placido rings (for a total of 1024 points) with a diameter ranging between 2.4 mm and 3.4 mm. Corneal curvature data are converted to corneal power by means of a keratometric refractive index of 1.3375. Additionally, photopic and mesopic pupil diameter and CD measurements are given.

Corneal topography enables the instrument to provide users with aberrometry of the anterior corneal surface, several keratometric indices (asphericity, spherical aberration, curvature irregularity and asymmetry) and keratoconus screening.

The Aladdin provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1, Olsen and SRK/T formulas, as well as toric calculation with Barrett’s calculator. For post-LASIK eyes, the Barrett True-K, Camellin-Calossi and Shammas No-History formulas are available.

**Zeiss IOLMaster 700**

The IOLMaster 700 is based on SS-OCT with a 1050 mm laser infrared light, which uses OCT in 6 scan lines at 0, 30, 60 90, 120, and 150 degrees to measure AL, ACD, LT, and CCT. With each scan, the device automatically acquires 3 B-scans of the cornea in 6 meridians, so that 18 measurements are taken altogether. Standard K is derived from 15 telecentric measurements of 3 rings, whose diameter varies slightly depending on the corneal curvature (for an average cornea with a radius of 7.7 mm, they are approximately 1.5, 2.5 and 3.3. mm). The average values obtained with this method are systematically identical to those taken at 2.5 mm with the IOLMaster 500. The 1.3375 keratometric index is used to convert curvature into dioptric values. The IOLMaster can also measure the posterior corneal curvature: for this purpose, it first builds a toric anterior surface model from the telecentric 3-zone K and then measures pachymetry using SS-OCT in the 6 meridians. The pachymetry values are fitted to the anterior surface model to create the toric posterior surface model. From the anterior and posterior corneal curvatures, as well as corneal thickness, the IOLMaster can also calculate (by means of a thick lens formula) the total corneal power, defined as Total Keratometry (TK). On the contrary of the other biometers calculating the total corneal power, the TK values are adjusted with a proprietary method to match, on average, the K values, so that existing IOL calculation formulas and IOL constants can be adopted with no changes.

The IOLMaster provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1 and SRK/T formulas, with either K and TK values. Toric calculation are performed with Barrett’s calculator and the Haigis suite. For post-LASIK eyes, the Barrett True-K, Haigis-L and Shammas No-History formulas are available.

Finally, the IOLMaster 700 is the only device showing the OCT of the fovea, thus enabling fixation control.

**Ziemer Galilei G6**

The Galilei G6 combines an optical biometer, based on PCI, and a dual Scheimpflug camera. It AL and LT by means of an 880 nm A-scan interferometer. Based on a proprietary algorithm, it derives anterior segment measurements (anterior and posterior corneal curvature, corneal thickness, and diameter) from the images obtained by 20 Placido rings and 2 oppositely rotating Scheimpflug cameras. The ACD is measured by means of the Scheimpflug cameras. Corneal power values are calculated based on the anterior (and posterior) corneal curvatures in the 1.0 to 4.0 mm central zone.

The following corneal powers are generated:

Simulated K: this is calculated by converting the measured radius of the corneal surface into diopters (D) using the standard 1.3375 keratometric refractive index.

Total Corneal Power (TCP) 1: this is calculated using the corneal index of refraction (n = 1.376). For the purpose of determining the focal length, the defined reference plane is the anterior corneal surface.

TCP 2: this is calculated using the aqueous index of refraction (n = 1.336). For the purpose of determining the focal length, the defined reference plane is also the anterior corneal surface.

TCP-IOL: this is calculated using the aqueous index of refraction (n = 1.336). For the purpose of determining the focal length, the defined reference plane is the posterior corneal surface.

TCP values are lower than Simulated K and therefore cannot be entered in any IOL power formula, unless specific constant optimization is carried out.

All topographic and tomographic measurements and maps of the Galilei G4 are available, including aberrometry, corneal thickness progression report and Santiago PTA report.

The Galilei G6 provides IOL power calculation by the Barrett II Universal, Haigis, Hoffer Q, Holladay 1 and SRK/T formulas (with a constant optimization tool), as well as toric calculation with Barrett’s calculator. For post-LASIK eyes, the Barrett True-K and Shammas No-History formulas are available. Moreover, Okulix software can be installed to calculate the IOL power by ray-tracing.