Parameters	N-BK7	H-LAK72	Difference Explanation
Refractive Index n(d)	1.51680	1.71300	H-LAK72 is approximately 13% higher
Abbe number Vd	64.2	51.81	N-BK7 has lower dispersion
Glass Code	517642	713518	/

N-BK7 has a refractive index of 1.5168, which is typical for standard crown glass, and has replaced traditional lead-arsenic
glass. Its Abbe number of 64.2 indicates low dispersion in the visible light spectrum, making it suitable for imaging systems
that require high color fidelity.

The refractive index of H-LAK72, 1.713, is significantly higher than that of N-BK7 and belongs to the lanthanum crown glass
family. A direct benefit of this high refractive index is that, at the same optical power, the curvature of the lens surface can
be flatter, thereby reducing the contribution of spherical aberration.

1.2 Transmission Spectrum Range

The transmission range of N-BK7 spans from 350 nm to 2.5 μm, with good transmittance from the near-ultraviolet to the
short-wave infrared. This makes N-BK7 a versatile choice for broadband systems—its transmission window covers a wide
range of applications, including visible-light imaging, fluorescence excitation, and near-infrared sensing.

As a lanthanum crown glass, H-LAK72’s primary advantages lie in the visible to near-infrared spectrum; its transmittance
drops significantly at the ultraviolet end, making it unsuitable for deep-ultraviolet applications.

1.3 Reflection Loss

At 587.6 nm, the single-sided reflection loss of N-BK7 is approximately 4.05%, and the total loss for both sides is
approximately 8.1%. For uncoated components, the Fresnel reflection loss at the air-glass interface is significant;
this is why most precision optical components require anti-reflection coatings.

2. Comparison of Physical and Thermal Properties

Parameters	N-BK7	H-LAK72
Density (g/cm³)	2.51	~3.01
Coefficient of thermal expansion (10⁻⁶/K)	7.1	~8.0
Glass transition temperature Tg (°C)	557	644
Rockwell hardness HK	610	514

Thermal Expansion and Temperature Stability: N-BK7 has a coefficient of thermal expansion of 7.1×10⁻⁶/K,
exhibiting minimal dimensional changes across different temperature ranges. The phase transition temperature of
H-LAK72 (644°C) is significantly higher than that of N-BK7 (557°C), providing better structural stability in high-temperature
operating environments.

Machinability: N-BK7 has a Knoop hardness of 610, which is higher than H-LAK72’s 514. This means that N-BK7 is easier
to polish to a high-quality finish, resulting in a higher machining yield. This is one of the sources of N-BK7’s cost advantage
in mass production.

3. Dispersion Characteristics and the Impact of Apochromatic Design

3.1 The Meaning of the Abbe Number

The Abbe number (Vd) is defined as: Vd = (nd - 1) / (nF - nC)
where nF and nC are the refractive indices at wavelengths of 486.1 nm and 656.3 nm, respectively.
N-BK7 has an Abbe number of 64.2, classifying it as a high-Abbe-number material with low dispersion; H-LAK72 has an
Abbe number of 51.81, classifying it as a medium-dispersion material.
In achromatic designs, it is typically necessary to pair high-refractive-index, low-Abbe-number materials with low-refractive-index,
high-Abbe-number materials to correct chromatic aberration. The dispersion characteristics of H-LAK72 give it certain advantages in
apochromatic designs.

3.2 Partial Dispersion Deviation

From a materials genomics perspective, comparing Abbe numbers alone is insufficient to evaluate a material’s performance
in advanced chromatic aberration correction. The relative partial dispersion deviation parameter (ΔPΔP) of N-BK7 is a
critical input parameter in optical design, as it directly determines the difficulty of correcting secondary spectral aberrations.

4.. Material Selection Recommendations

Select N-BK7:

The system’s operating wavelength range covers visible light to the near-infrared (350 nm–2.5 μm)
High requirements for dispersion control
Cost-sensitive, requiring large-scale processing with high yield rates
Components are relatively large, with weight reduction requirements

Select H-LAK72:

High optical power must be achieved within a limited space
Stringent requirements for spherical aberration correction
The system operates in high-temperature environments (higher Tg)
Design flexibility is constrained by refractive index

The difference between H-LAK72 and N-BK7 essentially lies in the positioning of the two material systems:
N-BK7 represents a “general-purpose” solution characterized by low dispersion, a wide bandwidth, and a mature
manufacturing process; H-LAK72, on the other hand, achieves a performance leap in terms of refractive index through
the introduction of lanthanide elements, providing greater design flexibility for high-performance optical systems.
The key to selecting the appropriate material lies in precisely matching the material parameters to the system’s design objectives.