Guidelines for the cover glass of the VL53L5CX Time-of-Flight 8x8 multizone sensor with wide field of view

Introduction

The aim of this application note is to provide guidelines for industrial design and how to assess cover glass quality. It details ST's recommendations on cover glass selection and design requirements for minimizing the crosstalk and optimizing the system.

Figure 1. VL53L5CX ranging sensor module: A 3D isometric view of the VL53L5CX ranging sensor module, showing its compact rectangular form factor with visible optical elements on the top surface.

General Information

The VL53L5CX is a Time-of-Flight (ToF) 8x8 multizone sensor with wide field of view (FoV).

The cover glass typically serves as an opaque window with a coating layer featuring apertures for IR light emission and reception. These apertures can be single oval or dual circular. Often, cover glasses include an IR filter coating on the underside.

The cover glass has two primary functions:

Physical protection of the device, including dust ingress prevention.
Optical filtering.

Aesthetic purposes may also influence cover glass design, allowing for dual apertures on the coating layer. The receiver aperture can be smaller if needed.

The air gap, the space between the cover glass and the module, is critical. Experimental data indicates that increasing the air gap size leads to increased crosstalk signal and signal loss.

System Crosstalk

The VL53L5CX, with its wide FoV, captures significant signal from the target but also experiences crosstalk. Crosstalk is defined as light from the module's emitter that does not reflect off the target but reaches the receiver via alternative, undesirable paths. This signal provides no useful target information (distance, reflectance) and must be minimized.

Crosstalk is influenced by optical setup, cover glass geometry, and properties, and can change over time due to scratches or dirt.

Figure 2. Crosstalk critical paths: A diagram illustrating optical paths in a Time-of-Flight sensor system. It shows the emitter (Tx) and receiver (Rx) FoV cones, the target, the cover glass, and the air gap. Paths labeled (1) represent the direct target signal, (2) represent crosstalk through the cover glass, and (3) represent crosstalk through the air gap.

The goal is to minimize crosstalk and maximize the target signal, avoiding obstacles or attenuation.

Increasing cover glass thickness generally increases crosstalk. Using the thinnest possible cover glass is recommended. A light-blocker can help break crosstalk paths through the cover glass.

Reducing the air gap also reduces crosstalk. A gasket is recommended to break crosstalk paths through the air gap, especially for air gaps > 0.7 mm, to keep crosstalk below the 100 kcps limit.

Negative impacts of crosstalk include increased signal loss, ranging non-linearity, and ranging standard deviation.

Crosstalk signal is temperature-dependent, generally increasing with temperature rise.

Figure 3. VL53L5CX final application configuration: Three diagrams showing different application configurations for the VL53L5CX sensor module: 1. Free air gap configuration: Shows the ToF module, air gap, cover glass, and outer cover without any sealing. 2. Gasket configuration: Shows the addition of a gasket within the air gap to seal the module and cover glass. 3. Gasket and light blocker configuration: Shows the use of both a gasket and a light blocker material within the air gap.

VL53L5CX Crosstalk Immunity

The VL53L5CX driver includes a calibration function for crosstalk compensation. This calibration, performed at the customer production line, compensates for part-to-part cover glass variations. Calibration data must be loaded into the module at each startup via the dedicated driver function.

Cover Glass Design

Cover glass design and structure significantly impact crosstalk. Manufacturing properties like internal particles, crystal defects, surface topography, and roughness contribute to light scattering and crosstalk.

To minimize light scattering and crosstalk, cover glass should be:

Free from defects in crystal structure or surface layer.
Free from internal impurities or dislocations.
Free from smudges or superficial artifacts.

Figure 4. Light scattering examples due to internal defects or superficial roughness: Illustrations depicting how internal defects (particles, crystal defects) and surface roughness in a cover glass can cause light scattering, leading to increased crosstalk.

Optical Transmission

The cover glass must allow transmission of IR light (940 nm, 1.6 nm FWHM) emitted by the VCSEL and received by the SPAD array. Optical transmission must exceed 87% within this bandwidth.

Table 1. Evolution of maximum ranging distance
Transmittance [%]	Estimated maximum ranging distance [mm] (1)
100	4000 mm
90	3800 mm
80	3600 mm
70	3400 mm
50	3000 mm
20	2400 mm

(1) Example of 4x4 mode, dark conditions, white 88% target reflectance, 30 Hz ranging frequency, with default driver settings

Signals not transmitted are lost or contribute to crosstalk, affecting performance and maximum ranging distance. High cover glass transmittance is recommended.

Cover Glass Coating

Cover glasses are coated for various purposes:

Colored ink for aesthetics.
IR filter to block unwanted IR light.
ARC: anti-reflective coating to reduce surface reflectance.
AFC: anti-fingerprint coating for fingerprint protection.

Superficial coatings can create optical paths for crosstalk. It is advisable to avoid coatings in exclusion areas, or use outer coatings that do not degrade fingerprint immunity (e.g., anti-fingerprint or anti-reflective coatings).

Haze

Haze is the percentage of light deviating more than 2.5 degrees from the incident beam. Crosstalk increases with the square of the haze percentile. Haze should be less than 2% (1% for 940 nm IR).

Figure 5. Representation of light haze definition: A diagram illustrating the definition of haze. It shows an incident beam passing through a material, with a portion deviating by an angle greater than 2.5 degrees, representing haze.

Cover Glass Tilt and Surface Parallelism

Cover glass surfaces should be parallel to the device surface to minimize crosstalk. If mechanical constraints require tilt, the maximum crosstalk must remain below 100 kcps. Recommended maximum tilt is ±10°.

Cover Glass Materials

Single-material cover glass is recommended to avoid performance alterations or increased internal light scattering. Suggested materials include Glass, Sapphire glass, Polymethyl methacrylate (PMMA), and Polycarbonate.

Cover Glass Mechanical Guidelines

This section details geometrical dimensions for calculating minimal aperture dimensions for cover glass coating.

Key dimensions:

Receiver mechanical aperture: circular, 0.51 mm diameter (0.4086 mm²).
Emitter mechanical aperture: rectangular, 0.72 mm width x 0.80 mm height (0.576 mm²).
Distance between optical emitter and receiver centers: 4 mm.

A single large aperture or two separate apertures can be used; the latter may offer better crosstalk immunity, especially without a gasket.

Figure 6. VL53L5CX mechanical dimensions: A diagram showing the mechanical dimensions of the VL53L5CX module. It includes dimensions for the optical centers, the receiver aperture (circular), and the emitter aperture (rectangular), along with overall module dimensions.

Note: Apertures must be aligned with the VCSEL optical centers, not mechanical centers.

Figure 7. Example of cover glass coating with single aperture: An illustration of a cover glass with a single aperture design, showing how rectangular apertures (aR, bR for receiver; aT, bT for transmitter) are defined and how their diagonals (dR, dT) relate to circular apertures.

Collector exclusion cones (61° along y, 55.5° along x) are used to calculate minimum cover glass apertures.

Figure 8. Cover glass Tx aperture in x direction: A diagram illustrating the transmitter (Tx) aperture calculation in the x-direction, showing the cover glass, air gap, and the Tx exclusion cone angle (55.5°). It depicts how the aperture size is determined based on this cone.

Figure 9. Cover glass Tx aperture in y direction: A diagram illustrating the transmitter (Tx) aperture calculation in the y-direction, showing the cover glass, air gap, and the Tx exclusion cone angle (61°). It depicts how the aperture size is determined based on this cone.

Formulas are provided for calculating aperture dimensions (aT, bT, dT, aR, bR, dR) based on exclusion cones, air gap (ag), and cover glass height (hcg). Formulas for calculating overall aperture width (W) and length (L) for a single aperture design are also provided, including tolerances.

Table 2. Cover glass dimension calculation
air gap	aT	bT	dT	aR	bR	dR	W	L
0	1.1890	1.3312	1.7849	0.9996	1.1192	1.5006	6.0428	2.1849
0.15	1.3469	1.5080	2.0219	1.1575	1.2959	1.7376	6.2797	2.4219
0.2	1.3995	1.5669	2.1009	1.2101	1.3548	1.8165	6.3587	2.5009
0.3	1.5047	1.6847	2.2588	1.3153	1.4726	1.9745	6.5167	2.6588
0.4	1.6099	1.8025	2.4168	1.4205	1.5904	2.1325	6.6746	2.8168
0.5	1.7152	1.9203	2.5747	1.5258	1.7082	2.2904	6.8326	2.9747
0.8	2.0308	2.2737	3.0486	1.8414	2.0617	2.7643	7.3065	3.4486
1	2.2413	2.5093	3.3645	2.0519	2.2973	308002	7.6224	3.7645

Dimensions assume a cover glass thickness 0.5 mm and the stated dimension is on the top side of the glass. This calculation includes 2° of angular tolerance (@tolerance) in addition to the collector exclusion cone (see figure below), then the calculation results are reported in Table 3. Cover glass calculation with 2 degree tolerance.

Figure 10. Cover glass Tx aperture with angle tolerance in x direction: A diagram showing the transmitter (Tx) aperture with an additional angular tolerance (2°) in the x-direction, illustrating how this tolerance affects the calculated aperture size.

Figure 11. Cover glass Tx aperture with angle tolerance in y direction: A diagram showing the transmitter (Tx) aperture with an additional angular tolerance (2°) in the y-direction, illustrating how this tolerance affects the calculated aperture size.

Table 3. Cover glass calculation with 2 degree tolerance
air gap	aT	bT	dT	aR	aR	bR	dR	W	L
0	1.2399	1.3849	1.8589	1.1643	1.0424	1.1643	1.5628	6.1108	2.2589
0.15	1.4045	1.5688	2.1056	1.3482	1.2070	1.3482	1.8095	6.3576	2.5056
0.2	1.4593	1.6301	2.1879	1.4094	1.2618	1.4094	1.8918	6.4398	2.5879
0.3	1.5690	1.7526	2.3524	1.5320	1.3715	1.5320	2.0563	6.6043	2.7524
0.4	1.6788	1.8752	2.5169	1.6546	1.4813	1.6546	2.2208	6.7688	2.9169
0.5	1.7885	1.9977	2.6814	1.7771	1.5910	1.7771	2.3853	6.9333	3.0814
0.8	2.1177	2.3654	3.1749	2.1448	1.9202	2.1448	2.8788	7.4268	3.5749
1.0	2.3371	2.6105	3.5039	2.3899	2.1396	2.3899	3.2078	7.7558	3.9039

Dimensions assume a cover glass thickness 0.5 mm and the stated dimension is on the top side of the glass. If the cover window is not parallel to the VL53L5CX module surface, then some pitch or roll may occur as shown in the figure below.

Figure 12. Cover glass pitch or roll rotation: Two diagrams illustrating the effect of pitch and roll rotation of the cover window relative to the VL53L5CX module surface. It shows how these rotations can alter the effective aperture shape and size.

Figure 13. Aperture shapes with cover glass rotation: Illustrations showing how cover glass rotation (pitch and roll) changes the shape of the apertures, impacting the calculation of their dimensions.

Crosstalk Compensation

Crosstalk compensation is a firmware feature that uses characterization and calibration data to mitigate the crosstalk effect. Crosstalk characterization is detailed in the VL53L5CX user manual (UM2884).

Lower crosstalk and less variation (due to smudge/haze) make compensation easier. Poor cover glass quality increases crosstalk. Smudge or haze degrades the target vs. crosstalk signal ratio.

Figure 14. Range vs target distance for high level of crosstalk: A graph plotting ranging distance against target distance for a scenario with high levels of crosstalk. The dashed line represents the ideal, zero-error range. The plot shows significant deviation from the ideal line at shorter distances due to high crosstalk.

Figure 15. Range vs target distance for low level of crosstalk: A graph plotting ranging distance against target distance for a scenario with low levels of crosstalk. The plot shows much better linearity and less deviation from the ideal line, especially at shorter distances, indicating effective crosstalk compensation.

Gaskets

Gaskets reduce crosstalk by minimizing reflections. An ideal gasket fills the air gap, has apertures for Tx/Rx cones, and forms a light barrier between channels, covering maximum area without impeding keepout zones.

Conclusion and Summary Table

Air gap size and cover glass properties significantly influence crosstalk levels. An air gap < 0.4 mm is recommended. For larger air gaps, a gasket may be necessary.

Table 4. Cover glass guidelines and summary table
Parameter	Recommended spec for maximum performances
Max crosstalk signal level accepted	100 kcps (max)
Transmittance at 940 nm	>87%
Transmittance haze (visible)	< 2%
Transmittance haze (IR)	< 1%
Air gap (1)	Without gasket	< 0.4 mm
Air gap + cover glass thickness	<1.5 mm
Cover glass tilt	±10°(2)
Number of cover glass apertures	Two circular holes are preferable to protect the light trap

Note: 1. Increased air gap potentially adds crosstalk. The crosstalk may be limited with the use of a gasket. Air gaps <0.4 mm keeps crosstalk below the recommended limits. Air gaps >0.7 mm require a gasket to remain within the 100 kcps crosstalk limit. 2. Assembly tolerance is ±2°

Figures above are for the final cover glass including any coatings applied. For a particular turnkey cover glass made by third party, contact your ST sales office.

Acronyms and Abbreviations

Table 5. Acronyms and abbreviation
Acronym/abbreviation	Definition
AFC	anti fingerprint coating
ARC	anti reflective coating
cps	photon count per second
FoV	field of view
FWHM	full width at half maximum
IR	infrared
PMMA	polymethyl methacrylate
Rx	receiver
SPAD	single photon avalanche diode
ToF	Time-of-Flight
Tx	transmitter
VCSEL	vertical-cavity surface-emitting laser

Revision History

Table 6. Document revision history
Date	Version	Changes
21-Nov-2022	1	Initial release

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