Today we speak about the Lord Voldemort of our industry. The light leak/ bleed/ 漏光 (Lòuguāng)!
Why I decided to make this post?
Every now and then people ask why light leaks are so present in every day displays and if they can be at eliminated fully during production?
How do we know?
We had plenty of challenges with Eve V and rejected quite a lot of screen batches due to light leaks and other defects.
But first. What are the most common screen defects and what are they?
- Light leak
- IPS glow
- Dead/Stuck pixels
- Pressure stains
In this post I’ll talk about pressure stains and light leaks and if you’d like it will be a pleasure for me to tell about other defects in the next post!
What is light leak?
Light leak/ bleed is backlight shining through where it is not supposed to. Most often it can be seen on edges of the screen and in some cases as smaller spots in various areas.
What is it caused by?
Two main reasons:
- Misalignment of LCM layers
- Pressure applied during assembly
1. Slight misalignment of the layers that go into making an LCD.
LCD panel has a lot of layers to create that beautiful image(I won’t go into detail on each of them as it would require a separate post).
Picture of all layers
Backlight is at the back of the screen and it allows you to see the actual picture as otherwise you would have to use a flashlight to see anything:) if one of the layers is slightly misaligned from the edge it creates light leakage in that area.
It’s quite easy to get few screens with perfect alignment of all the layers but it is much harder when they are produced in tens of thousands.
This depends a lot on the capability of the manufacturer and their clients accepted manufacturing tolerances.
Other common types of light leak?
Pressure stains/ pressure caused light leak
looks like a small bright spot or some times similar to a normal light leak and are caused by some internal components like screws or sharp board edges in the monitor/tablet/phone pressing against some of the panel layers causing them to stretch and letting more light to go through. In some cases pressure applied at the factory by workers during assembly can cause a pressure stain.
This is a much more common problem for phones and slim laptops as manufacturers chasing every 0,1 of millimeter of thickness often times don’t consider manufacturing tolerances of all the components or temperature change for metal components and their behaviour.
So often times pressure stain is not a defect from a screen supplier like LG or Sharp but rather a design mistake or acceptance of too big of a tolerance for incoming component quality check.
How does it actually happen?
Let’s say I am a factory producing for BRAND R. My client wants to see the slimmest laptop possible at the best possible price point. We lock down dimensions of all the components and their tolerances that go into a laptop. During assembly the factory accidentally uses screws that are a a bit too long to go into assembly of a laptop back causing increased applied pressure on the screen that at time of quality check doesn’t show any problems but during shipment to the end customer problem appears as the pressure is applied for too long time.
There are other issues as well but they occur more seldom and typically manufacturers cover those.
So why when buying a monitor you are the unlucky one with a light leak?
Quality standard and cost is the answer!
Typical QC document:
Posted this for illustration purposes. Let me know if you would like to see the one we use as it’s 35 pages long:D
All of these specs can be tweaked as per client requirement. Typically all defects are categorised by importance.
Critical Issue (CR) - is the Top Priority shipping blocker kind of issue. If Major issue is discovered production stops and root cause is investigated. Example would be panel setting itself on
Quoting the Standard:
Safety issues, such as fire, smoke, electric shock, burns, scratches and other endangering personal safety issues; Seriously affect the usage, such as no display, often automatically crashes / shutdown / blue screen / restart; Damage that cannot be restored, such as parts burned, cannot boot cannot enter into the operating system (OS);
Major Issue (MA) - Severe light leak, cracks, bent surfaces and too big gaps.
Quoting the Standard:
Electronic/structural function is not achieved, such as display isabnormal, speaker output sound distorted, battery cannot be charged,the expansion interface /UI no function, buttons no function, LED indicator lights’ status is incorrect, imperfect paintwork etc.There is bias on electronic/structural function, such as the display brightness is not up to the standard, speaker volume is soft, structure gap exceeds the standard etc.
Minor Issue (MI) is a defect that customer might not notice or not encounter or has a very minor effect on the use.
Quoting the Standard:
It’s a judged as a defect, but the end user may or may not encounter it.There is little or almost no impact on end user and the customer may accept this defect. The problem is superficial or tiny, almost no effect on function, and will not affect the user experience, such as prompt information is not accurate, wrong character on non-important parts etc
Most factories handling LCD displays are capable of identifying 100% of defects! Why defected goods end up on store shelves?
Cost is the answer.
Let’s imagine I am a screen producer.
- I produce 100,000 screens for my customer.
- 1 screen costs 100 USD without defect rate considered.
- from 1,00,000 screens 50% have no visually identifiable light leak from a typical end user viewing distance. 25% have minor light leak that can be seen. 20% have average light leak that most people would find annoying. 5% have SEVERE light leak.
As a customer I have few options:
- Tell manufacturer to scrap 50% of defected screens making my panel cost 200 USD
- Accept only minor light leak (25%) making my cost 133 USD per panel (25% of screens get rejected)
- Accept everything except critical issues making my cost 105 USD
Now let’s calculate the difference for each case:
- 100,000 x 200 (cost after defect rate) - 100,000 x 100 (what would my cost be with min standard) = 10,000,000 = 10 million USD.
- 100,000 x 133 (cost after defect rate) - 100,000 x 100 (what would my cost be with min standard) = 3,300,000 = 3,3 million USD.
- 100,000 x 105 (cost after defect rate) - 100,000 x 100 (what would my cost be with min standard) = 500,000 = 0,5 million USD.
So above is the problem! Good brand would have to pay double (in our case 10 million usd extra to achieve exceptional quality) a more typical brand would pay only 0,5 million USD.
*Some of you might think that calculation above is unfair as it does not include RMA (consumer return) costs. While it’s true! In the end of the day not so many people go for service and often times light leak is a grey zone in warranty of the product. *
The key question for the brands is how to make the correct decision?!
Let’s say I am brand A and I have no defect policy my price is 30-40% more than brand B that offers EXACTLY same spec but every second product will have an issue.
Which one would you choose?
- I would gamble and go with Brand B
- I would play it safe and go with Brand A
In the end of the day it’s all about willingness of customer to pay extra and competition’s offering!
What are your thoughts on this?!