Side Fingerprint vs Under Display Readers: What to Know

Side-mounted fingerprint sensors and under-display optical readers are the two dominant fingerprint authentication methods in modern smartphones. The side sensor is a physical capacitive button, while the under-display reader is an optical scanner built into the screen. Your choice depends on your priorities: speed and reliability favor the side sensor, while a clean, uninterrupted display favors the under-display reader.

Background: How We Got Here

Fingerprint authentication on phones began with dedicated capacitive sensors on the front or back. The iPhone 5s (2013) popularized the front home-button sensor, and many Android phones placed sensors on the rear. As bezels shrank and screens grew, manufacturers moved the sensor to the side, often combining it with the power button. This design became common on mid-range and some flagship devices.

Under-display readers emerged around 2018 as a way to achieve bezel-less, notch-free designs. The first generation used optical technology, projecting light onto your finger and capturing the reflection. Later, ultrasonic sensors (like those from Qualcomm) used sound waves to map the fingerprint in 3D. Both approaches eliminate the need for a physical button on the front or side.

Today, side sensors are still widely used in budget and mid-range phones, while under-display readers are standard on most flagships. The choice has become a trade-off between cost, design, and performance.

How Side Sensors Work

A side-mounted fingerprint sensor is a capacitive reader, the same technology used in older front and rear sensors. Your finger’s ridges and valleys create tiny differences in electrical charge when they touch the sensor. The sensor measures these differences and creates a fingerprint image. This image is compared to stored templates.

Capacitive sensors are fast and reliable because they read the fingerprint directly from the skin’s surface. They work well even with slightly wet or dirty fingers, and they don’t require the screen to be on. The sensor is always active, so you can unlock the phone by simply pressing the button, even if the display is off.

Side sensors are typically placed on the right or left edge, within easy reach of your thumb or index finger. They double as the power button, so you get two functions in one physical control. This placement is particularly convenient when the phone is lying on a table, as you can reach it without lifting the device.

How Under-Display Readers Work

Under-display optical readers work by shining a bright light (usually from the screen itself) onto your finger. The light reflects off your fingerprint ridges and valleys, and a tiny camera or sensor beneath the display captures the reflected image. The phone then processes this image and compares it to stored templates.

Optical readers are the most common type in under-display sensors. They are relatively inexpensive and work well in most conditions. However, they can be fooled by high-resolution photos of your fingerprint, and they may struggle in very bright sunlight or with very dry fingers.

Ultrasonic readers, on the other hand, use sound waves to map the fingerprint. They send ultrasonic pulses through the glass, and the echoes create a 3D image of the fingerprint’s ridges and pores. This makes them more secure than optical readers, as they are harder to fool with a 2D image. They also work better with wet or oily fingers, and they can even work through some screen protectors. However, they are more expensive and less common.

Speed and Reliability: Which Is Faster?

In general, side-mounted capacitive sensors are faster and more reliable than under-display optical readers. A capacitive sensor can read your fingerprint in about 0.2 seconds, while an optical reader takes around 0.3 to 0.5 seconds. The difference is noticeable in daily use, especially when you’re in a hurry.

Reliability also favors the side sensor. Capacitive sensors have a higher success rate on the first attempt, typically over 95%. Optical readers can fail more often, especially if your finger is wet, dirty, or the screen has a screen protector with a matte finish. Ultrasonic readers are more reliable than optical but still not as consistent as capacitive.

One advantage of under-display readers is that they work when the screen is off, but you have to tap the screen first to wake it up. Some phones allow you to tap a specific area to wake and read, but it’s still an extra step compared to just pressing the side button.

Design and Ergonomics: What Feels Better?

Under-display readers allow for a completely flat, uninterrupted display. There’s no notch, cutout, or bezel to accommodate a sensor. This is the main reason flagship phones use them. The design looks cleaner and more modern, and it allows for larger screens in the same body size.

Side sensors, by contrast, require a physical button on the edge. This adds a visible element to the phone’s side, but it also provides a tactile reference point. Many users find the side button easier to locate without looking, especially when the phone is in a pocket or on a table. It also serves as a power button, so you don’t need a separate button.

Ergonomics depend on your hand size and grip. Side sensors are usually positioned at the natural resting place of your thumb or index finger. However, if you’re left-handed and the sensor is on the right side, it can be awkward. Under-display readers are always centered at the bottom of the screen, which is easy to reach with either thumb, but you have to press on the glass, which some people find less satisfying than a physical button.

Common Problems and Misconceptions

One common misconception is that under-display readers are less secure than side sensors. In reality, both are quite secure for everyday use. Optical readers can be fooled by a good photo, but most phones have liveness detection that prevents this. Ultrasonic readers are even more secure. Side sensors are also secure, though they can be fooled by a 3D-printed mold of your fingerprint.

Another issue is screen protectors. Under-display optical readers often fail with thick or matte screen protectors. You may need to use a specific type of protector designed for the sensor. Ultrasonic readers are more tolerant but still may have issues with very thick glass. Side sensors are unaffected by screen protectors, as they are separate from the display.

Wet or sweaty fingers are a common problem for optical readers. The moisture can distort the light reflection, causing a failed read. Side sensors handle moisture better, but they can still fail if your fingers are extremely wet. Ultrasonic readers are the best for wet conditions.

Cost and Availability

Under-display readers are more expensive to implement than side sensors. The optical module and the software required to process the image add to the manufacturing cost. This is why budget and mid-range phones often use side sensors, while flagships use under-display readers. As the technology matures, the cost difference is narrowing, but it still exists.

Side sensors are also easier to repair. If the sensor fails, it’s a separate component that can be replaced. Under-display readers are integrated into the display module, so a failure often means replacing the entire screen, which is more expensive. However, under-display failures are rare.

When choosing a phone, consider your budget and how much you value the design. If you want the latest flagship with a bezel-less display, you’ll likely get an under-display reader. If you’re on a budget or prefer a more reliable sensor, a side-mounted sensor is a solid choice.

How to Choose the Right One for You

To decide between the two, ask yourself these questions:

• Do you often unlock your phone with wet or sweaty hands? If so, a side sensor or an ultrasonic under-display reader is better.
• Do you use a screen protector? If yes, check if it’s compatible with the under-display reader. Side sensors are unaffected.
• Do you prefer a clean, uninterrupted screen? If yes, under-display is the way to go.
• Do you value speed and reliability over design? If yes, a side sensor is more dependable.
• Are you left-handed? If so, check the side sensor’s placement. Some phones allow you to remap it, but not all.

In practice, most users adapt to either type within a few days. The key is to test the sensor in a store if possible, with your own finger, to see which feels more natural.

Frequently Asked Questions

Can under-display readers work with screen protectors? Yes, but not all. Optical readers often fail with thick or matte protectors. You need a protector that is specifically designed for under-display sensors, usually with a cutout or a special coating. Ultrasonic readers are more compatible but may still have issues with very thick glass. Always check the phone’s manual or the protector’s packaging.

Which is more secure: side or under-display? Both are secure for everyday use. Ultrasonic under-display readers are the most secure because they create a 3D map of your fingerprint. Optical readers can be fooled by a high-resolution photo, but liveness detection usually prevents this. Side sensors are capacitive and are also secure, though they can be fooled by a 3D-printed mold.

Why do some phones have both a side sensor and an under-display reader? Some phones, especially foldables, have both to allow unlocking in different modes. For example, when the phone is folded, the side sensor is accessible, and when it’s unfolded, the under-display reader is used. This is rare, but it exists.

Do under-display readers work in bright sunlight? Optical readers can struggle in direct sunlight because the bright light can interfere with the sensor’s ability to read the reflection. Ultrasonic readers are less affected. If you’re outdoors often, a side sensor or an ultrasonic reader is a better choice.

Can I register multiple fingerprints? Yes, both types allow you to register multiple fingerprints, usually up to 5 or 10. This is useful for registering both thumbs and index fingers, or for adding a family member’s fingerprint.

What happens if the sensor fails? If a side sensor fails, you can usually still use a PIN or pattern to unlock the phone. If an under-display reader fails, you may need to replace the entire display module, which can be costly. However, failures are rare and often covered by warranty.

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