diff --git a/docs/CBOR.md b/docs/CBOR.md
index 3536f9d6..93f6ae24 100644
--- a/docs/CBOR.md
+++ b/docs/CBOR.md
@@ -155,6 +155,7 @@ See [DECTRIS documentation](https://github.com/dectris/documentation/tree/main/s
 | - d                         | float                 | resolution \[Angstrom\]                                                                                                           |                           |          |
 | - I                         | float                 | integrated intensity (photons)                                                                                                    |                           |          |
 | - bkg                       | float                 | mean background value (photons)                                                                                                   |                           |          |
+| - var_bkg                   | float                 | non-signal (background) part of sigma^2, carried to the merge (photons^2)                                                         |                           |          |
 | - sigma                     | float                 | standard deviation, estimated from counting statistics (photons)                                                                  |                           |          |
 | - image                     | float                 | image number (present for each spot)                                                                                              |                           |          |
 | - rp                        | float                 | Distance to Ewald sphere \[Angstrom^-1\]                                                                                          |                           |          |
diff --git a/frame_serialize/CBORStream2Deserializer.cpp b/frame_serialize/CBORStream2Deserializer.cpp
index 7acc12dd..6ba60651 100644
--- a/frame_serialize/CBORStream2Deserializer.cpp
+++ b/frame_serialize/CBORStream2Deserializer.cpp
@@ -509,6 +509,8 @@ namespace {
                 r.I = GetCBORFloat(map_value);
             else if (key == "bkg")
                 r.bkg = GetCBORFloat(map_value);
+            else if (key == "var_bkg")
+                r.var_bkg = GetCBORFloat(map_value);
             else if (key == "sigma")
                 r.sigma = GetCBORFloat(map_value);
             else if (key == "image")
diff --git a/frame_serialize/CBORStream2Serializer.cpp b/frame_serialize/CBORStream2Serializer.cpp
index 70e29d9f..21056641 100644
--- a/frame_serialize/CBORStream2Serializer.cpp
+++ b/frame_serialize/CBORStream2Serializer.cpp
@@ -267,6 +267,7 @@ inline void CBOR_ENC(CborEncoder &encoder, const Reflection& r) {
     CBOR_ENC(mapEncoder, "d", r.d);
     CBOR_ENC(mapEncoder, "I", r.I);
     CBOR_ENC(mapEncoder, "bkg", r.bkg);
+    CBOR_ENC(mapEncoder, "var_bkg", r.var_bkg);
     CBOR_ENC(mapEncoder, "sigma", r.sigma);
     CBOR_ENC(mapEncoder, "image", r.image_number);
     CBOR_ENC(mapEncoder, "rp", r.dist_ewald);
diff --git a/tests/CBORTest.cpp b/tests/CBORTest.cpp
index b7df0480..08be6b06 100644
--- a/tests/CBORTest.cpp
+++ b/tests/CBORTest.cpp
@@ -969,20 +969,35 @@ TEST_CASE("CBORSerialize_Image_Spots", "[CBOR]") {
 TEST_CASE("CBORSerialize_Image_Reflections") {
     // Prepare a few reflections with distinct values
     std::vector<Reflection> refs_in;
+    // Every float is given a distinct value: a field the encoder forgets reads back as 0 and fails
+    // here. var_bkg was added to Reflection and to the HDF5 writer but not to the CBOR map, so every
+    // broker-written file carried a zeroed background_variance until this test was widened to see it.
     refs_in.push_back(Reflection{
         .h = 1, .k = 0, .l = -1,
+        .image_number = 789.0f, .delta_phi_deg = 0.125f,
         .predicted_x = 1024.5f, .predicted_y = 768.25f,
-        .d = 2.345f
+        .observed_x = 1024.75f, .observed_y = 768.5f,
+        .d = 2.345f, .I = 1234.5f, .bkg = 3.25f, .var_bkg = 41.5f, .sigma = 37.125f,
+        .dist_ewald = 0.0025f, .rlp = 1.75f, .partiality = 0.875f, .zeta = 0.625f,
+        .image_scale_corr = 1.125f
     });
     refs_in.push_back(Reflection{
         .h = -3, .k = 2, .l = 7,
+        .image_number = 790.0f, .delta_phi_deg = -0.25f,
         .predicted_x = 5.0f, .predicted_y = 10.0f,
-        .d = 4.0f
+        .observed_x = 5.5f, .observed_y = 10.5f,
+        .d = 4.0f, .I = 12.0f, .bkg = 0.5f, .var_bkg = 6.75f, .sigma = 4.25f,
+        .dist_ewald = -0.001f, .rlp = 2.5f, .partiality = 0.5f, .zeta = 0.25f,
+        .image_scale_corr = 0.875f
     });
     refs_in.push_back(Reflection{
         .h = 0, .k = 0, .l = 1,
+        .image_number = 791.0f, .delta_phi_deg = 0.0f,
         .predicted_x = 0.0f, .predicted_y = 0.0f,
-        .d = 10.0f
+        .observed_x = 0.25f, .observed_y = 0.5f,
+        .d = 10.0f, .I = -1.5f, .bkg = 0.125f, .var_bkg = 0.875f, .sigma = 1.5f,
+        .dist_ewald = 0.0f, .rlp = 1.0f, .partiality = 1.0f, .zeta = 1.0f,
+        .image_scale_corr = 1.0f
     });
 
     // Minimal DataMessage carrying reflections
@@ -1020,8 +1035,21 @@ TEST_CASE("CBORSerialize_Image_Reflections") {
 
         CHECK(b.predicted_x == Catch::Approx(a.predicted_x).margin(1e-6f));
         CHECK(b.predicted_y == Catch::Approx(a.predicted_y).margin(1e-6f));
+        CHECK(b.observed_x == Catch::Approx(a.observed_x).margin(1e-6f));
+        CHECK(b.observed_y == Catch::Approx(a.observed_y).margin(1e-6f));
 
         CHECK(b.d == Catch::Approx(a.d).margin(1e-6f));
+        CHECK(b.image_number == Catch::Approx(a.image_number).margin(1e-6f));
+        CHECK(b.delta_phi_deg == Catch::Approx(a.delta_phi_deg).margin(1e-6f));
+        CHECK(b.I == Catch::Approx(a.I).margin(1e-6f));
+        CHECK(b.bkg == Catch::Approx(a.bkg).margin(1e-6f));
+        CHECK(b.var_bkg == Catch::Approx(a.var_bkg).margin(1e-6f));
+        CHECK(b.sigma == Catch::Approx(a.sigma).margin(1e-6f));
+        CHECK(b.dist_ewald == Catch::Approx(a.dist_ewald).margin(1e-6f));
+        CHECK(b.rlp == Catch::Approx(a.rlp).margin(1e-6f));
+        CHECK(b.partiality == Catch::Approx(a.partiality).margin(1e-6f));
+        CHECK(b.zeta == Catch::Approx(a.zeta).margin(1e-6f));
+        CHECK(b.image_scale_corr == Catch::Approx(a.image_scale_corr).margin(1e-6f));
     }
 }