# Chemistry and imaging on NextSeq 500/550
**Chemistry and base calling**
* The NextSeq 500/550 uses two-color chemistry, enabling very fast imaging.
* Signal in only the red channel results in a C call, whereas signal in only the green channel results in a T call.
* Signal in both channels yields an A call, and a G call results from the absence of signal in either channel.
* Because of this, there must be at least one non-G base in the first 5 cycles of sequencing for a cluster to be registered during template generation.
* Additionally, a G overcall in a run is generally indicative of a failure of clustering (or paired-end turnaround) or very low clustering.
**Reference:**
* [Two-Channel SBS Technology](https://www.illumina.com/science/technology/next-generation-sequencing/sequencing-technology/2-channel-sbs.html)
**Imaging**
Review the section titled 'Flow Cell Tiles' of the [NextSeq 500 and NextSeq 550 Sequencing System Guide](https://support.illumina.com/downloads/nextseq-500-550-systems-product-documentation.html).
**Camera numbering**
* The NextSeq 500/550 uses six cameras to image the flow cell.
* Cameras are numbered 1-6. Cameras 1-3 image lane one.
* Cameras 4-6 image lane three.
* After lanes 1 and 3 are imaged, the imaging module moves on the X-axis to image lanes 2 and 4.
**Lane numbering**
* Lanes 1 and 3, called lane pair A, are imaged at the same time.
* Lanes 2 and 4, called lane pair B, are imaged when imaging of lane pair A is complete.
* A Lane Pair A—Lanes 1 and 3.
* B Lane Pair B—Lanes 2 and 4.
**Swath numbering**
* Each lane is imaged in three swaths.
* Swaths are numbered 1-3 for high output flow cells.
**Tile numbering**
* There are 12 tiles in each swath of each camera segment.
* Tiles are numbered 01-12, regardless of swath number or camera segment, and represented in two digits.
* The complete tile number includes 5 digits to represent the location, as follows:
* Surface—1 represents the top surface; 2 represents the bottom surface.
* Swath—1, 2, or 3.
* Camera—1, 2, 3, 4, 5, or 6.
* Tile—01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 11, or 12.
Example: Tile number 12508 indicates top surface, swath 2, camera 5, and tile 8.
\
\
\
| |
| :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------: |
| *For any feedback or questions regarding this article (Illumina Knowledge Article #2078), contact Illumina Technical Support* [*techsupport@illumina.com*](mailto:techsupport@illumina.com?subject=Question%2FFeedback%20Regarding%20Illumina%20Knowledge%20Article%20#000002078%20-%20Instrumentation%20\&body=Dear%20Illumina%20Technical%20Support,%0D%0A%0D%0A)*.* |
---
# Agent Instructions: Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://knowledge.illumina.com/instrumentation/nextseq-500-550/instrumentation-nextseq-500-550-reference_material-list/000002078.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.